The Time they invest in the PS3 (Be it Low-Res Textures, Low Poly Models or other things) prevents them to go full rampage on FFXIV.
Converting thousands Textures into Low Resolution isn't done quickly.

The PS4 has enough RAM and CPU Power to handle High Poly Models and High-Res Textures. Yet the fact is, both the PC and the PS4 have troubles once an Area is overpopulated. (You cannot tell me you can keep 60 FPS in Idleshyre or Mor Dhona)

Dropping the PS3 Version (I highly doubt this will be bad for the PS3 Players, you can upgrade for free on another Version) will result in more Development Time, which can be used to either add more Gameplay or better Visuality.