In the hypothetical world where you are 98% sure that he turns into Kirk Cousins in year two, do you sign him? I think you obviously do, in large part because that's not a marginal improvement from Cam and it's a lot hard to get someone better than Kirk Cousins than everyone on this board seems to think.That's not really the right question, though. The right question is really "how often does giving big money to an average QB work out for that team"? For every Eli Manning (and even in that deal his big money didn't kick in until after 2011), there are like 50 Alex Smiths, Andy Daltons, Joe Flaccos, Jared Goffs, etc.
In any event the underlying point is incorrect - no team would ever spend a first-round pick on a QB the team thinks is only going to be "average" (with the possible exception of a good team drafting late in the first round that thinks they are just a QB away from serious contention). Certainly there are tons of first-round QBs who turn out to be average, but presumably they were all drafted with the hope/expectation they'd be considerably better than that.
Put another way, if BB and Pats look at Mac Jones and think "sure maybe this guy will turn out to be league-average, but we really think in our system with some coaching he has a chance to be much better than that," then the pick makes sense. But if they look at Mac Jones and think "there is very little chance this guy turns out to be anything more than league average" there is no way they are going to draft him anyway just because they feel like they have to marginally improve from Cam.