I'm posting this in this thread as well as I feel it to be relevant, and may explain how we (or more to the point Game Freak) have got to this point:
It's been extremely disappointing watching this shit-show unfold for the last four years or so, and something I've been apprehensive about during all of that time.
I think a lot of the most hardened fans among us knew something wasn't right in 2015 when Zygarde was beginning to get a lot of attention in the anime and across merchandise and spin-off games. A lot of people, myself included, suspected this was leading to a 'Pokemon Z' reveal but as time went on it became apparent that might not be happening.
We had X and Y in 2013, Omega Ruby and Alpha Sapphire in 2014, and many predicted 'Z' in 2015 or even 2016 after time passed. And it wasn't even just Zygarde that caused speculation for this, we had the likes of Ash-Greninja as well, a bizarre Mega Evolution that wasn't a Mega Evolution for Greninja, and has somehow made Ash canon in the games which largely remained separate in the past (the only other references that spring to mind are event Pokemon given away to tie in with the anime or movies).
Fast forward to February 2016 and Sun and Moon were revealed, and not only as part of a brand new seventh generation for Pokemon, but they'd also be coming to the 3DS. Prior to the DS, generations were typically one per console, however that's always been blurred line with the Gen 1 being on the Game Boy and Gen 2 being on the Game Boy Color (Gen 2 was fully playable on the original Game Boy barring Crystal), so it was perhaps unexpected 3DS was getting another generation, the DS was a massive hit sales wise and kind of made sense to have two generations given its lifespan and install base.
I think normally this would've been fine had we been following normal console cycles and the Wii U was a moderate success and perhaps even followed up with a more generic home console, and the same goes for the 3DS, it just so happened that the Switch being a thing threw a proverbial spanner in the works for how the Pokemon and console generations would pan out. So now we have the Switch (then known as the NX) on the horizon, and a new generation of Pokemon on the 3DS.
I remember there being speculation at the time that a Pokemon 'Z' was scrapped in favour of fast tracking a new generation of Pokemon for the 3DS. There is some evidence for this, such as the bizarre insertion of Zygarde into Sun and Moon (it has nothing to do with the mythos of Alola and the only thing we have close to an explanation is the appearance of Ultra Beasts as to why it's there) and a weird special form of Greninja with an Ash 'bond evolution' which to me always felt like a redesigned mega evolution that would've been for Greninja (and I imagine Delphox and Chesnaught would've also had one in a hypothetical Pokemon Z).
You can also see in the very design of Sun and Moon that these games are not really that suited to the 3DS, with the bottom screen being relegated to a very ugly, and actually irritable at time, Rotom map, which doesn't even function well as a map (i can't begin to describe how many times I've tried to navigate myself around Alola by trying to use it as a map and becoming utterly dumbfounded at why everything is so hard to read - this is even more ironic when you consider that given these games are in full 3D with larger environments than previous games, a map could be especially useful). We don't even need to go into the fact that the games are nothing pretty and at times with a lot going on can be a technical nightmare (although Sword and Shield haven't exactly done a lot to remedy that particular grievance lol).
It's my thinking that Sun and Moon were hurrily made for the 3DS after the Wii U wasn't being saved and needed to be followed up with the Switch sooner rather than later. Z was cancelled, a few of its elements were shoe horned into Sun and Moon, which in themselves were now being made for the 3DS rather than the Switch (which would've perhaps given us a 4 year long sixth generation, and a launch of Sun and Moon in the fall of 2017).
This is also evidenced in the fact that Ultra Sun and Ultra Moon did not do a lot to further the mythos of Alola, giving us 'powered up' versions - a few new Pokemon (a first for a mid-generation game) and a cool although slightly short post game quest involving Team Rainbow Rocket and previous villains from the series. I don't think they'd have been badly received had we got 'Pokemon Stars' or even a year off in between Sun and Moon USUM to let them have a little longer development time. In my opinion by today's standards they are certainly the most cash-grabby Pokemon games, even if they do have more content than the likes of Yellow and Crystal and other 'third versions'. The quick release was most unnecessary and the resources put into it could perhaps have gone towards the likes of Sword and Shield.
I always thought we'd get FireRed and Leaf Green anniversary remakes to celebrate 20 years of Pokemon, however I didn't think they'd do this in the form of Lets Go Pikachu and Eevee, which I think may have taken shape the way they did with the influence of Pokemon Go - had that not been as popular as it was we may have had a more traditional set of games. I would perhaps liked to have seen these on the 3DS as well - especially if we had Z in 2015, but given the way things played out I think if Sun and Moon had come to the Switch in 2017, followed by LGPE in 2018 as it happened, and then THIS YEAR, we had USUM, they still could've kept Pokemon yearly (which seems to be their desire) but each game could have breathed (and especially for SM and LGPE with all the confusion and misinformation as to whether they're part of the same generation or not) and had proper development time.
This then would've lead us to a more ambitious and properly cooked in the oven with all the trimmings Sword and Shield in either 2020, or even 2021, made sure everything was as good as it could be with a full roster of Pokemon, moves, overworld animations, GTS, and everything else that has upset people about these games. We then could've had Diamond and Pearl remakes the year following and perhaps 'Lets Go Johto' the year after to follow that sub-series.
(TLDR) But instead we've had rushed games, cash-grabby titles and a total miscommunication between the devs and fans.If only they made Pokemon Z eh, and this could've all been avoided.