Historically, backwards compatibility wasn't a given for console games. And when it was attempted it has either been imperfect, or discontinued.
If you want to keep playing a old console game, hang on to the hardware needed to play it.
I think Steam got it right when it came to software delivery (as did Apple's app store). It would make sense for the future to look more like those systems than having all the issues that go along with physical media.
- No more scratched discs
- No more "sold out" games
- No more paying for physical distribution, inventory management, and shelf space
I believe the fear that there will be draconian rules in place is unfounded. The "digital software stores" (like Apple, Steam, Microsoft, etc) have a fair set of rules. You can use your games on up to 5 devices (Apple, Microsoft) or unlimited devices (Steam). Sure, you can't re-sell your games, but you may be able to purchase said games for a lot less - see the game prices on Steam (when they're on sale) and Apple's app store as examples of this.
If Sony did choose to go with draconian rules, I suspect they'll just lose out in the market share wars. Competition is good, and all the players in this game will keep prices reasonable.