Think about what would've happened if they hadn't announced it then? It'd still be in development. They still would've made 2 Sequels to FF XIII and a bunch of Side titles after KH2. FF fans would be clamoring to Square Enix saying "OH MY GURD, WHY DO YOU KEEP RELEASING SEQUELS TO XIII, GIVE US A NEW NUMBERED TITLE ALREADY!" And the KH fans would be going, "WE'RE SICK OF SIDE TITLES, GIVE US KH FUGING III ALREADY MAN!"

 

Square would've had to give an answer, and either lie or tell the truth. And if they just said "Wait a little longer" and not give a legit reason for why they were doing those things, people would've gotten even more pissed. So really, they didn't really have a choice, since Nomura and Square are dragging their asses with XV. : It really wouldn't have made a difference on if they announced it in 2007 or not in the end, the secret would've been outted EVENTUALLY.

I'd be upset if I didn't already own the game

They were going to max out the PS3 (hence why it wasn't going to 360) so Square Enix decided to move it to next gen in 2011. This isn't going to be a simple PS3 to PS4 type game they did with a lot of launch games. This a whole new game, I'd nearly call it a remake of an unfinished game.