Didn't Sony release God of War 2 for the PS2 4 months after the PS3 released?
Plus isn't there a strong rumor that the next Call of Duty will be 8th gen only? I can't see this happening because it will loose over 10 million sales, but it's a rumor.
As for MGSV, Beyond, TLoU, GTA V, and other late comers to PS3, some of these games have 3 years or longer development times, PS4 dev kits have only been around for what 12-18 months? Devs can't just adapt their project to the next gen, and with 160 million PS3/360's, any game released on a 7th gen platform will surely sell better.
I am surprised that a lot of games aren't seeing ports on the PS4/Nextbox though, but I guess devs don't want to make it look like they aren't trying, and when a PS3 game is ported to the PS4, it just won't look as good as a dedicated PS4/High spec PC game like Killzone or Battlefield.
Either way, this just gives you time to think about which HD platform you're getting next gen, and lets their initial appeal/aticipation die off. It also gives you a chance to finish what you have right now, and not feel like you're playing an outdated device.