You're assuming MS DID get the code right..... Flight is SO much smoother with FAR more objects (trees etc) than FSX is with far fewer objects. This is a comparison with my current system (quad core 3.0, 560TI, 8 gigs of ram, 7200 HD etc. I don't know anything about code, but I can say that Flight's code seems to be much smoother.
If no backwards compatibility means smoother flying... so be it.