I also don't think FFXII has the same appeal as FFX, but it is somewhat close. It's total global sales were close to 6M iirc, which is about 75% of FFX sales.
But if anything, the FFX/X-2 HD collection showed that gamers are hungry for good FF games, and not only I believe having an HD version of XII would give a lot of gamers a chance to get a shot at the game for the first time, but I think it would also help SQEX fill the gap between now and FFXV, since they don't have anything announced yet between now and the release of FFXV.
About ports of the PSone games, I completely agree. I'm just a sucker for FFs and I would love to see HD graphics for those games. And now that you mention that you can already play all of those on other systems besides FFXII, that's the exact same reason Kitase mentioned when he said XII would be SQEX next port.