There will be a MGS5, just not directed by Kojima and not with Snake as the hero. It's been rumored it'll be a prequel with Big Boss, once more.
And I wouldn't count PS3 out just because of FF. It'll still be coming out for PS3 as well. And PS3 sales, while still in third, have been more than respectable in comparison to not only where the PS2 stood at this point in it's life post-release, but the 360 as well.
The Wii has destroyed both systems in sheer numbers sold. That doesn't make it much much better or make it so that the others will stop being made.
Also God of War is still out there for the PS3. Gran Turismo. Resistance could end up being a big franchise. Plus the Blu-Ray aspect will continue to be a selling point, which could lead to multi-platform titles becoming more popular on the PS3 down the road.
The PS3 isn't on the brink of extinction any more than DC is.