Why would SE make a new MMO? Just because it's been "X games since the last FF MMO" doesn't mean they're going to do it. FFXIV is doing more than well enough now that I can't see them spending the resources to make yet another game in a dying genre. A lot of repeat complainers here on the forums keep bringing up "old graphics and gameplay" dragging the game down, but if that was the case, then we wouldn't have an entire datacenter closed off and talks about increasing the server capacity.
According to a recent interview, Yoshi-P said he would only make a new MMO if he could make something good enough that would move all of the players over from FFXIV. He also said in the same interview that the FFXI sub base is actually growing and they plan to release content for its 20th anniversary.