HD textures is pretty much the last thing I'd care about, and I have a PC who could run everything on max settings perfectly. And while you are right that often it is just what players ASSUME to be the reason, there have been several instances where SE said "we can't do that because the PS3 wouldn't be able to handle it", which is probably the reason why now so many people jump to that as the standard reason, since they heard it so often before.
But, like Kuwagami said, even if the PS3 support was dropped, low-grade PCs would still be supported, one day the PS4 will be considered old, and so on. We just can't have nice things, can we?
