I disagree. If there were for example a mainline title that one didn't like, there may end up being the chance of having no other options for that gen from the mainline series. For example if single player titles are looked at for the PS3 gen, there was only the XIII trilogy. Keep in mind that tastes can vary greatly from person to person.
I'm aware of the FFXIII mess, but I would rather them not have newer titles constricted by old hardware, I would want the next title to top the last one in terms of scope and ambition doing things that weren't possible on older hardware this could actually force them to try harder to avoid another Final Fantasy XIII.
Keep in mind that there are also remakes, that could be coming out in that timeframe as well, so your single player experience won't just be FFXVI, it could be a FFVI Remake, a FFVIII Remake, or a FFXVI spin-off in the same vein as Versus XIII was to FFXIII, except on an engine that's not shit.
The way I would see it for Final Fantasy in the 9th generation:
Final Fantasy XVI, and two spin-offs based on the franchise similar to the FNC how it had Final Fantasy XIII, Agito XIII, and Versus XIII.
Final Fantasy VIII Remake
Final Fantasy VI Remake
Final Fantasy IX Remake
Dissidia Final Fantasy sequel
For the 10th Generation:
Final Fantasy 1 Remake
Final Fantasy 2 Remake
Final Fantasy XVII MMO
Final Fantasy XVIII Single player
Note: Exception here, because one is an MMO, and the other is single player