That whole point you are making is something that I strongly disagree with. You are comparing Prescott's rookie year with Mayfield's rookie year, and using some very slanted statistics, and outright mis-characterizations to reach incorrect conclusions.
Dak had a good rookie year, and played well, but through the draft, and with Romo's injury, he fell ***-backwards into an incredible supporting cast. He was given lots of credit for that, and regarded as one of the upcoming superstars in the NFL. When his supporting cast slipped the next two years, his performance slipped even more. Now he is regarded as an average starting QB--nothing more.
You are characterizing Dak as 'leading' the team to the winning records. That may have been true his rookie year, but it certainly mis-characterizes what happened this year. He was there, but he wasn't a main driver of their success. The offense averaged 21.2 pts/game--24th in the league. That was with two starting Pro Bowl offensive linemen, a Pro Bowl running back (who led the NFL in rushing), and a Pro Bowl WR. The team won 10 games this year with better defensive performance than offensive performance. The defense 'led' the team this year--not the offense, and certainly not Prescott.
The Browns averaged 22.4 pts/game this year--14th in the NFL, with both Mayfield and Taylor at QB. They were 0-1-1 the first two games under Taylor, and were behind 14-3 at the half in the third game when Mayfield came in. They came back and won that game, and even though Mayfield didn't start, it was far more due to Mayfield's play than Taylor's. So to give Mayfield proper credit, I would characterize his record as 7-7. This was taking over a team that had only won one of their last 34 games, and had 10 consecutive losing seasons, finishing last in their division most of those.
In Dak's one good season of play, the team improved by 9 games. You give Dak the credit in your post, but during that season, observers were split about who was more valuable--Prescott or Elliot.
This year Mayfield had one Pro Bowl lineman, and one Pro Bowl WR. The Browns averaged more points than the Cowboys, and did so without as good of a defense, and without as good of a supporting offensive cast around Mayfield as Prescott had with the Cowboys.
QBR is a very flawed statistic. It doesn't penalize a QB who holds the ball too long and takes a sack. That is a very negative play for the offense, but for the QBR it is better than throwing the ball away. It is an old trick for veteran QBs to run out of bounds 2 or 3 yards behind the line of scrimmage, and preserve their QBR, rather than throw the ball out of bounds. I've actually heard one QB talk about another QB doing it, so I know it happens. Does holding the ball too long and taking a sack sound familiar? It should.
Completion % is highly dependent on the play call, and how conservative a QB is. Are they legitimately trying to get a first down, with a lower probability play, or are they throwing it 4 yards short of the sticks on 3rd down, pumping up their stats, and giving up the ball? Does that sound familiar? It should.
PPG is a much harder statistic to manipulate. It also a much more reflective statistic of performance than the statistics you cite.
Even more important than statistics is the fact that I watched the players play. Dak is slower getting through his reads (IMO), and less accurate than Mayfield. And less accurate on easy throws by NFL standards.
As far as the receivers for the Browns, here is my take:
Njoku is basically the same player as Jarwin. They both run well, and tend to be better receivers than blockers.
Here are some comparative stats:
Njoku 871 snaps, 56 receptions, 639 yds, 4 TDs.
Jarwin 451 snaps, 27 receptions, 307 yds, 3 TDs.
Njoku is not as effective as Cleveland hoped when they drafted him. Cleveland spent a 1st round pick on a very similar player that Dallas got as an undrafted free agent. Neither is all that good of a TE--despite your characterization. Dallas wants to run the ball, so they expect a TE to throw a block every now and then. That limits Jarwin's playing time, and it would do the same thing to Njoku if he were in Dallas. Nobody who really understands what Dallas is trying to do on offense, and what Njoku provides, would kill to get him. That's for fans who see a former 1st round draft pick and assume he's a star waiting to happen in Dallas.
Callaway has talent but he drops the ball a lot, and at times can be both sloppy and lazy on his routes. Think Dez Bryant lazy after he didn't get a pass for two quarters. They want to upgrade from Callaway in Cleveland. Jarvis Landry is a good receiver, but he is not as good as Amari Cooper in my opinion.
The Jones family needs to be exceedingly careful about extending Prescott. This has the potential to really ruin their tenure as owners, and turn the fan base against the team. That was already happening this year, when Cooper joined the team, and they turned things around.
