PS4 limitations. That's what limit the most FFXIV right now. And don't get me wrong, I don't want to start a PC versus console war, but, at the moment, XIV can run on PS4, PS4 Pro and PC. PC can be far better than the PS4 Pro or even not announced yet PS5, for people who have lots of money to put in a good build. PS4 Pro is still decent at the moment and it will probably able to keep up for ShB and another expansion. After that it will probably be just PC and PS5. That's purely my guess, but if you remember well, before Stormblood, we had the PS3 limitation and it made a big difference when they removed the PS3 support. We were able to have larger inventory, more items in housing, better graphics, just to mention those.

The PS3 was released in 2006 and you were able to play until 2017 to XIV. The PS4 was release in 2013, and if you add ShB + another expansion, this bring us to 2023, so almost the same age than the PS3 when it was removed. I think that it's just a logical bet to say that the support for PS4 will ends with the release of the 7.0, and remove at the same time the limitations that we have right now.

Every other "limitations" are more like "choices" of where to put their ressources. Adding a new race means making or modifying every models of every existing gear + adding the new one. This takes a LOT of time, and this is the reason why there won't be any headgear visible for Viera at the launch, but Yoshi-P said it will come... one day. Also, maybe the male Viera will come, maybe not. They said they won't release more race to XIV, but maybe they can complete those 2 new races in the future. They also released only 2 jobs on an expansion because adding 3 is too much work.

As for the new MMORPG, FF XI will probably die in the next years, when they'll reach the "minimum active players limit" playing the game. Beyond a certain number of active player, the game won't be profitable enough for them to keep on running it. This will leave us with XIV only. Yoshi-P already said they can make at least 3 other expansion (excluding ShB), and they probably can do more, so I don't see XIV die in a close future. But, even if it's their engine, and even if you remove the PS4 limitations, we will eventually reach the limit of the XIV engine. Meaning, if they want to keep an MMO, they'll need to make a new one eventually. And people now wants awesome graphics, awesome gameplay, awesome stories and lots of content, well, making a new game like that takes years of developement.

To be honnest, I won't be surprise if FF XVII (maybe?) is in development at the moment and will be a future MMO that will release in 6 or 8 years from now. They know that they can run 2 MMOs at the same time. And this brings them a lot of money. So I don't see why they won't release a new one when XIV will be their only MMO and will be in his last years. But, that's just my opinion, they can also chose to give up on the MMO genre.