Think about this for one second: you think Microsoft and Sony will abandon all the millions and millions they have devoted into building the libraries of digital content (hundreds of games and possibly near 1000 counting indie/Minis, thousands of pieces of DLC, and other misc digital content) working through so many deals and negotiations with companies, all to throw it away because they have a new console?
It's pretty much 95% surely known they will be migrating the marketplaces to the next console. Retail disc game backward compatibility is still arguable, but I can't see them throwing that library away as well, there's just too much they've built up and especially now with online communities they can't be leaving them in the dust on the previous console. Who would buy their new console then?
Online connectivity and marketplaces have changed everything. We will be approaching a whole new kind of generation gap of consoles, one we have never seen before, because beforehand there was almost nothing tied to the consoles and people could leave them behind with no losses. Here on the other hand, migration is primary.