Here is what I think would make sense would happen, next gen consoles should be like PS4.5 or xb1.5, new machines with 100% backward compatibility, but faster CPU/GPU and maybe a bit more RAM, games compatible with both systems could be released on the same disc, they would just have pre-defined settings for each machine's computing power (equivalent to running the game on Ultra on PC then the same game on High or Medium on a lower specced PC).. or like on iOS where games just run worse on the older phones...
There are benefits to the platform owners to this, they have less risk of a client "defecting" to the other platform, since people get to keep their games... and well people get to keep their old games, just like on PC (until eventually the OS/hardware is no longer compatible with their old software)...
However, if the power difference is not dramatic some people may never upgrade, and this is OK as long as they keep buying new games.
So, 2 or 3 years AT MOST if I am right... I think MS is more likely to do so, however Sony had a good track record as far as backward compatibility goes until the PS4 was released with NONE of it :-/ so we shall see.
