I already tried Gaikai on my PC and I think the service is running really well. Fighting games could be a problem, but apart from that most games, especially in single player, should work pretty well with that.
PS3 BC also was one of the first things that popped into my mind when they acquired Gaikai and I think they could keep that platform alive pretty effectively if they made an iOs and Android app (PS3 controllers could be connected to many of the tablets/phones over bluetooth). PS3 Disc games could probably as you described only be played by a PS4 as they need to be verified first though.
Ofcourse there would have to be a monthly fee for using Gaikai, but I think if they combine it with the Plus subscription (like, an additional 3$/€ per month on top of the plus subscription to use it) that could work out nicely and give those people not using a PS4 enough of an incentive in my opinion.
