Even if the digital split is huge, I still don't think it'll reach XV and I definitely don't think it'll reach the wide new audience they were hoping for, unless some legs really hit it. Youtube views were relatively low during the whole marketing cycle and, while the demo seemed to be a hit, it feels like it may have been too little too late. I sincerely love the game, but the reactions to pretty much every showing have been muted in terms of the mainstream. It never seemed to hit how they wanted it to.
The funny thing is, with some feedback, I really think the something that builds on XVI could absolutely be *that* breakout game. There's a lot to love with XVI, a few stumblings aside. I'm absolutely loving my time with the game and I think its the best single player FF game since at least XII.
If I were to take a few guesses, I think the lack of a real party system hurt it. I know that's been my one gripe with the game, so take me with a grain of salt, but I legit think the ensemble cast is a marketing point in the series. You can't show a ton of heartwarming party interaction and cool gameplay with different characters in the trailers if its really focused on a single character and those moments don't exist much (and where they do exist are story spoilers so you can't put many of them in trailers). Think about when Barret or Tifa showed up and were seen as playable in the VII Remake trailers. People popped off. And nostalgia was a big thing for that, of course, but it was also just cool to see the different characters and their styles. Those hype moments couldn't really exist in FFXVI marketing because of its single character focus, and I do think an ensemble cast matters in these games.
Also, much as I personally like the setting, I think the "grounded" (lol to say that about the game having actually gone through it) medieval setting hurt it. It made sense when the game started development and Witcher 3 and GOT-Fever were everywhere, but by the time it released, those boons had faded. I think it's relatively clear that the outlandish character designs and crazy cheeseball tropes (I say it with all the love in the world for Final Fantasy cheese) are an appealing element to the series.
I really don't think the action combat or the lack of RPG elements (which is a criticism I pretty vehemently disagree with, at least in comparison to the rest of its own series) hurt it. FFXV had strong sales and was an action game. Honestly, the marketing of FFXVI as "the first action FF game" was fucking bizarre. I personally think FFXVI is a vastly better game overall than FFXV, but XV had an immediately emotional story hook with its characters at the forefront, weird and striking character design, an interesting (at least on a surface level) "modern fantasy" world, and, loathe as I am to say it, an open world.
And the above is why I think it would be a mistake to throw the baby out with the bathwater here. There's a very strong base to build on with XVI. Hell, it's at an 87 on Metacritic and an 89 on opencritic. That itself indicates a very good game, if not necessarily an all-timer. And CBU3 is very, very good at hearing feedback.
You keep the fantastic action combat, expand the ability customization, put together a party of three or four cool, outlandishly designed characters that I can build with those options, even out the structure of the game (seriously, the game is so backloaded in quality it's crazy), and I really think you could have a hit. A really emotional, high concept story by Natsuko Ishikawa would help too. Also, I really, really hate to say it, but open world, or at least much larger open zones, may really be a factor here. Which annoys me to no end but I can't deny it.
CBU3 *can* make this happen, I think. But you have to let them build on what they've created here rather than starting from scratch for every goddamn game in the series.