No the point that was proven was Dak is 20+ point points over replacement.
It's not that he's better than one of the best backups in the league, it's that he's light-years better than said backup.
And if there is a QB that valuable you sign that player to a long term deal. Anything else is wrong.
Thank you. It is important to point out, like you indicated, Dak Prescott is at least 20+ points better than the BEST backup in the NFL. Those were their words: "Andy Dalton is the best back up QB in the NFL and Dallas is no worse with Dalton than we were with Dak." Of course, many others said that Dalton was as good or even better than Dak, but that is a different story for a different day.
Well, the best back up in the NFL at QB has proven to be about 5 standard deviations downward from Dak. If the BEST backup is so far removed from what you have. You pay what you already have.
If Aaron Donald has the best back up in the NFL, yet his back up produces 1/5 what he does when he is forced into duty because Donald is not there...all the more reason you PAY Aaron Donald.