We're not talking about KH3 here. These are just remasters of old games; they don't need to be on the PS4.
People who are getting rid of their PS3s are going to be in a minority. You can either get rid of your PS3 and skip out on the collections or keep it for the collections and your old games.
Just because it doesn't need the power of the PS4 doesn't mean it shouldn't be available on a current, new system though. Madden doesn't
need to be on PS4 but it will I'm sure. Same for CoD.
Those aren't remasters but in that case KH1 and 2 don't need all the power available on PS3 either. It looks about as good as it could possibly look (except fish mouths I guess) and all they added was a higher resolution, higher-res textures, and some higher polycounts on certain character models. I'm sure the PS3 could do even better.
No harm putting the same title on PS4 as well so everyone can play it, especially since there's a good chance Kingdom Hearts 3 itself will be on PS4. It's just weird to me to put out two collections on an old console, which are leading up to a game that you'll need a new system for. And if you get that new system you can't play the two collections on it. Hopefully with the new architecture it won't be that difficult to port. Maybe they can make a single two-disc collection with both games on it for PS4.
With the lack of backwards compatibility on PS4 I think we might see more than a few games with separate PS3/PS4 versions to improve what would normally be a harsh transition from the old console to the new.