Its not a question of how much revenue FF14 generates. Its a question of how they allocate that revenue.

I suspect that a significant chunk of the revenue goes towards their "general funds" rather than "FF14 operating funds" or "FF14 development funds" ... and from "general funds" goes to FF15 or some other game that SE is developing. An increase in FF14 revenue is likely to largely go towards "general funds" rather than "FF14 development funds".