Theoretically it could take care of PS3 BC, and PS5 giving people games stronger than the PS4 system can do by itself at the end of its lifespan. Isn't that the whole point of cloud computing?
The big question mark is how they monetize it, how all the various publishers get their cut and if this is going to be appealing to consumers at all. I'd rather have physical BC, but I'm still kind of curious to see how they utilize Gaikai. They're going to use it for something. Timed trials, PS+, demos? Who knows.
i dont think PS3 BC through Gaikai is possible at all... not just because all of that - publishers, rights, cost of servers/bandwith/maintainance, etc, simply because it would require PS3 for every instance of game being played. Thats just too big of a cost to start with for a service that doesnt have a future.
I bet what they will do for Gaikai is PS4 games and remastered PS3 games as a service not to just PS4 owners but to PS3, Xperia, Vario, Bravia owners who dont want to buy PS4, just want to play games.
If I was Sony, I would build it into every TV that they sell, every Vaio, every Xperia... and they sell a lot of those things, a lot more than just their consoles.
They will probably "re-master" most popular PS3 games for PS4 though, add better textures, better resolution, basically make them perfect and sell them at reduced price and offer them via Gaikai.