What everyone seems to worry about here is that Sony can use this against piracy and/or game resale. Going streaming-only is a long-term direction that, if implemented right, could well improve Sony's bottom line and introducing it gradually is a great means for them to ensure gamers don't simply take their business elsewhere when it is the only option left.
Take the ability to stream older PS1-PS3 titles. Old titles can be a source of free money for a publisher like Sony but due to those titles' smaller size they are especially easy to pirate (according a torrent tracker the entire PSN PSX collection takes up around 58.7 GiB; that's 179 game titles for the PlayStation). Streaming would eliminate the possibility of this kind of piracy.
Still, I think you can make a strong argument that Sony won't implement a stream-only policy for games this generation since in the US, a key market, far too many of gamers just don't have the bandwidth yet.
>-- rolling video replay of your screen as a core system service
You've got to note that this feature will be "free" in terms of computational resources for every streamed game.