Let me exaggerate the scenario and take a step back and look at the cap in 2009.
It would be a tough argument to put a better player above Sidney Crosby, and a tougher argument to justify paying other people more money.
Flashforward to a little more recently... is Benn an amazing player? Yes. Is Subban? Tavares? Yes. Yes.
Are they better than Crosby, no (same may argue though). Are they better now than Crosby was then? Definitely no.
Are they making more on their most recent contracts than Crosby?
Yes.
But the cap rose, and so max. contracts rose, and cap hit percentages rose.
Assuming the cap keeps rising, it's inevitable that lesser player than McDavid will eventually make more money than him, with a smaller initial cap percentage hit.
Maybe not Matthews, but its not a stretch for it to be him either.