I don't think it's a matter of dropping PS3 entirely, at least not for several years yet. It is quite possible, however, that there may start being more and more features that work only on PS4 and PC.

There are already a few differences:
  • PS3 only supports cross-hotbars where PS4 and PC have both regular straight hotbars and the cross-hotbars.
  • On PS3, when you're in a FATE area, the FATE objectives take the place of your quest tracker. On PS4 and PC, you see both.
  • There was one of the UI features that was added to all except the PS3 version. IIRC I think it was the option for the larger inventory/retainer inventory windows that display four "pages" of inventory at a time.
There's probably a few other differences I can't think of off-hand. Over time, there will likely be more.

As for basic content, we know they'll be supporting PS3 at least through the first expansion which is coming out next year. They hadn't decided how far past that they would continue supporting it, at least as of the last time they told us those plans (which was several months ago).

Even if at some point down the road they decide to not release a later expansion on PS3, it's not as though that would stop PS3 players from continuing with all the content that's already out. They just wouldn't get the option of buying additions to it without upgrading to a different platform. Which is pretty much where the PS2 version of FFXI is at now. You can still play FFXI on PS2, but can't get the latest expansion for it (at least not for the NA/EU version).

It's theoretically possible that they could go beyond that and eventually drop PS3 entirely, so that that platform no longer gets the software updates needed to log into the game, but I highly doubt that will ever happen. It's far more likely that "dropping PS3 support" would just mean no longer releasing expansions for it, but it would still keep getting all the patches needed to play the basic game.