The thing is that the PS2 -> PS3 generation marked a large point of transition for Japanese game developers, leading to high production costs and lower than expected returns on the investments made in JRPG's. This meant that home consoles were no longer the hub of JRPG's that they were prior to this point of transition.
The genre largely migrated to the PSP/3DS or portable dedicated gaming platforms where the expectations for JRPGs was significantly lower to a similar product on home consoles. This made the cost to production to profit much more appealing to JRPG developers.
This is where we see a split when it comes to JRPG's, Premium JRPGs (console JRPGs) and "pocket" JRPGs (any JRPG released on dedicated handheld gaming platforms (no Mobiles don't count) such as the PSP/V and the 3/DS/New family.
The market for JRPGs isn't as concentrated as it was in the PS 1 and 2 days, is there still going to be a market for premium JRPG's by the time FFXVI comes out? I honestly think that the current model for what defines premium JRPGs will become unsustainable by the time FFXVI happens if and when it does.
So what does that have to do with FFXVI?
Simple, diminishing returns. FFXVI will be much more expensive to develop then FFXV, and the amount of sales needed to justify the games development along with the chance that more of the market for premium JRPG's could very will shrink from what it is now.
SE knows this, that's why they're using FFXV to bring in as much "new blood" to the fan base as they can. The amount of new blood that FFXV brings in as stated by Tabata I believe will be what decides if a FFXVI even ever happens.
I believe that FFXVI if it happens will be a drastic departure from what we are seeing from FFXV, FFXVI will probably come on the PS5 at the earliest and will be pursuing a radically different direction to FFXV both in style, presentation and substance.
We are seeing a sort of Renaissance for JRPG's as the definition of what constitutes a premium JRPG is being challenged by Underdog JRPG's, JRPG's that are built on a smaller scale and require a much lower financial commitment from the developer.
We probably entering a generation where FF as a brand will probably only release a single mainline title per generation if not once every two generations, by the time FFXVI comes out it should be possible to completely realism photo realism and it will cease to be unique at that point.
The alternative is that FFXVI is a game built on a much smaller budget that depends on other factors to gain consumer interest.
FFXVI won't have the sales numbers of FFXV I guarantee you that, but at the same time it won't be developed with the same budget as FFXV and will be taking a radically different route to FFXV and probably require much less sales to be considered successful.
Lets say that FFXV needs 5 million in sales in order to be a success, FFXVI would probably only need about 2 million at that point in order to be as successful. By the time FFXVI even comes out, technology would have progressed to the point that hyper realism would become the new "brown" of the PS360 generation.
Sorry if i went off topic, I useally have a hard time putting my thoughts into words