There's a whole other thread discussing the importance of BC.
What demo's are you playing?!
Largest demo I've come across is maybe 2 GBs. 7-15 GBs is more likely the entire game.
There's a whole thread discussing that too, but as Sony were never selling PSN content as being forwards compatible any more than they've sold any other content as being forward compatible, there's no reason for consumers to assume their PS3 content should work on PS4, nor reason to think content sold as part of a network service or play-anywhere service like PSMobile wouldn't be trusted. And even then it can't be trusted, because they may remove content from the streaming service one day. Whatever Sony do with this, they can communicate the message effectively.
I also question whether this is any good for BC. What hardware are Sony going to run their PS3 games on to stream? The mother of all emulators on a truly monster PC rig? Old IBM Cell servers going cheap? Or a PS3 for each user? It won't be cost effective. Just let anyone wanting to play PS3 games use their existing PS3 until such a time as PS3 emulation becomes as cheap as PS1 emulation is now and offer people a nostalgia trip.