expandability? What and hamstring newer model sales?
If this was feasible Microsoft and Sony would love it. New model sales cost them money, as they're sold at a loss. Expandability would likely be sold at a nice profit and keep the costly need for new model development off in the horizon. It's actually exactly what Microsoft and Sony would prefer.
But, as history shows, it's a complete failure and it never, ever works. The N64s expansion was the only one that at all caught on, even then barely, and mostly because it was included with its killer app - GoldenEye.
Also, Tumbler, none of those differences are functional. No game requires a bigger HDD on the PS3, just enough HDD. On the 360 games don't require one at all, and this is something that ended up biting Microsoft in the ass. Backwards compatibility isn't a functional difference for new games, only old.