Well its not really a sim if all D-F players are not going to be able to improve/achieve and compete at the top end.
I agree the majority will be nothing more than scrubs for their career, but with the right coaching, tactics .. whatever etc, ANY player should be able to progress to the top level.
What I am saying is, if I take a F rated potential running back - blast 2000 yards + per season with him, break records etc... will he be rated as highly as an A rated potential player who did the same or worse on the field?
Thats the key point, not that 'the real NFL' said so.