That's actually incorrect. Contracts are based more on their ranking in the league. I've done long-term sims using XP sliders all at 25%....upper-echelon QBs with OVRs in the high 60s are making exorbitant money....and it's true across the board. It's the one thing Madden does that actually makes a lot of sense....
I think that a lot of what you see later on in franchise is due more to CPU teams using a "Stars and Scrubs" approach. Very few CPU teams carry high-priced depth at any position...backups tend to be players on rookie contracts or really crummy vets. It's a pretty smart approach IMO (one the NFL has been trending towards in the last 2 decades IRL) and while it may result in lower team OVRs, their top-end talent is still usually quite competitive.