I think theres a middle ground here and you and others are talking about 2 slightly different things. The average backup QB does not make or justify 10M per. A handful of borderline backup/starter QBs like MM or Tyrod Taylor or Teddy Bridewater could demand 10M per.
18 qbs are set to make 10M or more next year. Once Alex Smith and Teddy Bridgewater are cut, Big Ben restuctures and Drew brees retires .. there will be 14 QBs making 10M or more next year.
NFL Rankings
No = back up Qbs dont make on average 10M per year.
Yes = a good backup will cost you close to 10M per.