Yes, AD is worth more than a bad, easily replaceable QB. He isn't worth as much as a good, hard to replace QB. I'd put the percentage as more than 1/4 of them, but you seem to be agreeing with the issue that by virtue of their playing position, QBs are more valuable than DTs, and that it doesn't take an elite QB to be more valuable than any DT.
Nobody here is saying that Ryan Fitzpatrick should be paid more than AD, just that Donald doesn't have the game value of good to very good QBs, nor merit being paid as such.
Off the immediate topic, but I was reading on a website that the Browns have $69 million in cap space. So they could pay Donald and have the total contract be manageable in future seasons. Not that they should, but they do have AD's former DC there, AND have a need to show strong improvement this season.