It's technically perfectly feasible. They just need to put the Cell and RSX into a MCM or even better a single die with a proper die shrink (I think the PS3 Super Slim right now already uses 32nm lithography processes, which is very small). Blu-Ray drive, HDD, RAM etc. can be used with the PS4 hardware. I'd say if those rumors are true and Sony beats MS in terms of release date and offers PS3 hardware backward compatibility, then they already have a huge advantage.
The NextBox sounds like it's gonna be an incredibly expensive piece of hardware anyway, if they truly want to add Kinect 2.0, as we all know is quite pricey.