I feel like this could go either way. Sony invested a ton of money into Gaikai in order to get PS3 games playable via PSNow. If you had asked this question a year or two ago, I would say that there was no chance in the world of them making PS3 games playable outside of this.
However, with PSNow spreading to more and more platforms (PC, TV, Blu Ray players) I think formal PS3 emulation on PS4 could be a good move for them. If nothing else it would certainly keep the "hardcore" gaming fans happy and more likely to stay with Playstation. I think a move like this would depend on how successful PSNow is via the other platforms I mentioned above.
You don't understand. It's not that Sony doesn't want to, Sony doesn't know how. To emulate hardware properly traditionally require a system two gens newer. Short of putting a PS3 chip insider PS4 there is no way to do this.