All Sensors that work with the Bayer-Matrix will give similar performance if the pixel size is similar, and the quality of the materials used. You could build a crappy sensor with the same specs as high-end one, but no serious company would dare to do so
HOWEVER, companies will not necessarily stay with the Bayer-Matrix way of doing it .. so there comes improvement probably, in particular regarding initial sharpness (before any camera software messes with the data) and hopefully someday regarding dynamical range.
Also the amplifier just behind the sensor can make a huge difference in particular regarding noise.
So here we are, the camera body is quite important and expensive these days, certainly more expensive than film-bodies. However, even if you go for the high end DSLRs still when you buy adequate glass for them, the latter ruins your bank account!
On my DSLR I can clearly see the weak points of my lenses (whcih are really not cheap ones). So what is the point of investing huge sums of money on the body if the lenses downgrade your output?
I personally am much more relaxed when I pay a lot for a good lens, than if I pay alot of money for a camera body which will be obsolete in 2 years time as technology has advanced.
These days a good lens will be your friend for quite some years, and it will probably survive several camera bodies.
And please don'T forget, that sensor materials degrade over the years ... diffusion processes will kill any sensor at some point. So today's cameras are not built for eternity anyway.

Therefore I really like the concept of a digital back which you can replace after some years without replacing the whole camera (if those Hasselblad digibacks weren't just so hell expensive

).