I knew someone would bring up Tannehill, which I think reinforces the idea that quarterbacks really don't get that much better elsewhere. We're talking about the best case scenario of the last decade, out of all the quarterbacks that got traded into what looked like a better system.
But even with that best case scenario in Tannehill, there's still some brake pumping. He's playing with an outlier running back who is by far the best in the league and arguably the only one capable of consistently setting up the passing game with dominant running. Tannehill is still throwing for less than 250 ypg. The team ranked 30th in passing percentage ratio, and the two teams below them (Baltimore, New England) have a by-design running QB element.
The Titans have been at a clear disadvantage when matched against the real elite QB led competition. And as you alluded to, he was already an average (perhaps slightly above average) quarterback by most measures in Miami. I have little doubt if the Titans had signed Brady this offseason, they'd be champions right now.