Pay him and pray.
I can't think of a scenario where some college QB today or any other QB that would be available would be more trustworthy and deserving of it. There is significant risk, he's not a complete QB. It's a team game, and I'm betting his issues this year have a lot more to do with the team around him than himself.
To be clear, I'm scared like hell, but you have to do it and he deserves it. I had more of a knot in my stomach when we gave Flacco his deal, even though he had just won a SB. I knew deep down that was a high water mark by far, and he was far more average. With Lamar, I have a LOT of confidence that he'll be just fine.
My fear really comes from the way the salary cap works in the NFL, and how paying one person that much can really impact things. But that isn't on Lamar, that's a league wide NFL salary cap thing. Every team worry's about that. If you pay two high end defensive players big deals in one off-season, they have to hit too. Sometimes they don't, you see it all the time all over the league.
I agree with you. I was happy as hell to pay Flacco but pretty much knew he wasn't ever going to be in the MVP race and mostly wouldn't be in the pro bowl discussion (still surprised he never went to one). But you see teams fail to find a QB time and time again so you just hold on tight when you get one, cause if you don't have one you have no chance. If you happen to find one whose ceiling is best player in the league and has reached that ceiling, how can you let that go? You only do if you think he's no longer capable of that, and I simply don't. It's definitely scary and risky though, because if you're wrong you're simply wasting several years, just like Stanley, Peters, and Thomas count 30M against the cap for nothing this year.
Flacco's contract after the first year (which is often low) was anywhere from 10-15% of the cap. Allen's is more like 15-20% (An aside, I haven't watched any Bills games this year but Allen is a whole lot worse than he was last year). Lamar next year will be at 11% so we'll start to see the effect of that in team building. But I would pay him, because I think the main issues are temporary and fixable and the ceiling is still best player in the league. Like you said, pay and pray.