I'm a little late to this discussion. I was referred to it the other day but I haven't gotten around to replying because of lack of time. Apologies if it's considered gauche here at ERA to bump a thread that's been inactive for a week.
The changes in sales curves this generation are entirely explicable as being the result of the same forces that influence the sales curves of every other system. The timing and degree of price cuts, the effects of new models (be they form factor changes or full spec upgrades), and the effects of major system-selling software. Systems used to have a clear growth-peak-decline pattern, but as Mr. Piscatella points out that's no longer the case.
It used to be that systems would have a pronounced peak in sales somewhere in their first three years on the market.
Last generation, we saw greatly delayed peaks, with the 360 and PS3 peaking in 2011, their sixth and fifth full years, respectively. The Wii had a fairly normal curve for a Nintendo system, peaking in its second full year, but for conventional consoles we ended up with a protracted generation.
Why did this happen? Well, I think the best explanation is because of pricing. Last generation, price cuts for the PS3 & 360 were fewer in number, smaller in terms of percentage drop (and in the 360's case in absolute dollar terms), and spaced out more overall, resulting in systems that declined in price much more slowly and spent most of their lives being more expensive than systems in prior generations.
To keep a long story short, the 360 & PS3 both started off relatively weak and experienced slower growth than the PS2 did, not reaching peak sales until their respective slimline models were issued well into their lifespans. The 360 S produced a significant increase in 360 sales, making 2010 & 2011 a rather pronounced peak period, but a belated one. The PS3 actually exhibited clearer growth, going from very poor prior to its cut to $400, to being able to about match the 360 during the Jan.-Aug. period of 2008, to growing even more once the Slim was released. The PS3's peak was more of a plateau, with 2011 being only slightly higher than 2010 (2009 was only as high as it was thanks to the last four months of that year when the Slim gave its initial boost).
Now, what about the current generation? The PS4 & XBO have spent most of their lives cheaper than the 360 & PS3 in inflation-adjusted terms. As a result, aligned sales have strongly favored the PS4 & XBO (note that the third graph aligns the launches of the PS3 & 360):
But you'll notice that the bulk of the growth, the sharpest increases in the surpluses, have been in the holidays. The PS4 & XBO's combined surplus increased by more than 2.5 million units in the 2016 holiday period alone.
This is because sales have experienced a significant shift towards the holidays this generation thanks to official temporary Black Friday price cuts. While that trend began in 2011, it was solidified this generation. These big Black Friday deals deals have greatly diminished the effects of the permanent price cuts the systems have gotten. It has also resulted in consistently strong holiday sales, with combined PS4+XBO sales in Q4 far exceeding 5 million units for the past three years (and this year may make it a fourth). Compare this to combined PS2+Xbox sales, which only crossed the 5M mark in Q4 once (in 2002), and combined PS3+360 sales, which only did so twice (in 2010 & 2011).
The net effect of this is Gen 8 being more Q4-centric than ever, and total Q1-Q3 sales seeing far less variation than before. The Q1-Q3 period of 2015 saw combined PS4+XBO sales grow less than 3% YoY, while for the same periods in 2016 & 2017 were down about 5% each year. This year has seen Q1-Q3 sales grow something like 17% YoY, which is okay, but is only about 6% higher than Q1-Q3 2015. The biggest variations we've seen are in the holidays, with Q4 2015 being up 30% from Q4 2014 (though that was because Sony didn't offer any big discounts for the PS4 in the 2014 holiday season). Q4 2016 was down nearly 10% YoY, and Q4 2017 nearly matched Q4 2016 (though if you ignore October, the Nov.+Dec. period of 2017 was up about 4% YoY, not a huge difference but it's there).
The siphoning off of Q1-Q3 sales by Q4 (and esp. November) has also resulted in combined PS4+XBO sales in the non-holiday months just barely keeping pace with combined PS3+360 sales. This generation has looked big and healthy for conventional consoles compared to last gen, but you can thank Black Friday for that.
As for this year, the PS4's YoY growth can be attributed entirely to software. God of War, Spider-Man, and Red Dead all moved a good amount of hardware. But ignore April, September, and October, and we see baseline sales that have actually dipped ever so slightly. The Slim & Pro did help out a bit in 2017 (Q1-Q3 sales were up 17-18% YoY), but didn't produce a clear peak, and their upwards effect on sales has clearly ended. 2018 was the Year of the System Seller for the PS4 in the U.S.
As for the XBO, the One X has clearly helped out, with significant YoY improvements this year from a rather mediocre 2017. But again it hasn't produced a clear peak, and Q1-Q3 sales this year were weaker than those in 2015, and roughly flat with 2016. Also, September suggested that the One X effect might be coming to an end, with flat sales YoY for the month (October had Red Dead, which was also a system-seller for the XBO, so we can't make a determination).
The Pro and One X are unlikely to produce further sales growth unless Sony & MS introduce deep price cuts with the intent of having them fully supplant the slimline models. Q1 2019 will be the first big test for PS4 & XBO sales with current models at current prices. If we see both systems down noticeably, then that will likely signal them entering the terminal decline period of their lives. It's possible that deep cuts to the Pro and One X could drive some more sales growth, but with Gen 9 hardware known to be in the works and likely due for release in 2020, it's questionable if the Pro & One X will even get permanent price cuts of any kind. The Pro didn't even get a temporary cut for BF this year. And it's possible that such a cut might not result in any significant growth, as not everyone is likely to upgrade even at a reduced price. Eventually, no amount of attempts to stimulate sales will keep a system's sales from irreversibly declining.
In any case, the effects of the Pro and One X are overstated, and hardware revisions in general haven't had the effects they used to, just like how permanent price cuts haven't had the effects they used to. Without a breakdown between sales of the two, it's difficult to separate the effects of the Slim and Pro for the PS4, but between they two they failed to produce the kind of growth we saw with the PS3 Slim. The XBO S absolutely failed to produce significant growth past its first three months, and while the One X did help make Q1 this year the XBO's best Q1 to date and make Q2 its second-best Q2, it too was nothing like the 360 S in terms of what it did for sales.
Between the shift of a greater portion of annual sales to Q4 thanks to temporary BF price cuts and the relative failure of hardware revisions to produce strong growth, it's no wonder sales curves have been wonky this generation. But nevertheless the console cycle will continue unabated. The PS4 & XBO are at over 48M combined LTD, likely to be north of 53M by year's end. There is a maximum addressable market, with only so many households willing, able, and planning to buy a current-gen console at some point, and eventually Sony & MS will run out of potential customers to sell PS4s & XBOs to. The systems will enter the terminal decline phase of their lives before too long, regardless of their subversion of the usual "growth" and "peak" periods. By the end of 2011, the 360 & PS3 had a combined LTD total of 52.65M units, and in 2012 they began their declines. Since the PS4 & XBO will be at about that point by the end of this year, it wouldn't be a stretch to assume that they're likely going to start declining soon, probably next year, as the number of first-time buyers starts to dwindle. They'll probably end up at about the same ~70M units combined the PS3 & 360 ended up at, maybe slightly higher due to population growth potentially increasing the overall size of the addressable market as well as the potential for more mid-gen upgrades as usual since the Pro & One X were full spec upgrades (the first for home consoles since the Japan-only SuperGrafx) instead of just changes in form factor.
Well, that took longer and went on longer than I wanted, but I wanted to cover all my bases. Sorry if it sounded a bit ramble-y. Hope you all enjoyed the read and my attempts as sales analysis.