The Bills have a generational talent at QB, a brand new stadium, and a bunch of decent players on the roster. There's more than enough to work with. The QB alone would make this the #1 available job. It's almost never the case that a HC or GM job opens when there is a superstar QB in his prime. Why? Because a superstar QB in his prime=consistent winning.
Which means the most important ingredient is not the GM or HC in that scenario. Beane being replaced is not going to mean missing the playoffs.
Zach Taylor, the most overrated HC in the NFL, got to a Super Bowl because of Joe Burrow, 2 WR and the DC who got blamed once the cheap owner let all the talent walk. Look at him now in Indy. Funny how that works.
Players win games, and having one of the best QBs in the NFL means this team has a very high floor. The HC raises the ceiling.
McDermott allowed Daboll to be Daboll and Allen to throw the ball all over the place and be unstoppable, so while he was uncomfortable with it I think that might have had more to do with personal stuff and not wanting to feel like second fiddle on his own coaching staff. As a younger coach at the time (experience wise) I can sort of understand that. Offense is unstoppable, everyone loves "Dabs" and you're kind of the grumpy parent telling everyone to stop having so much fun.
Maybe it would be different now, because McDermott is more established and a little bit less tight and serious.
I think you have a conversation as ownership first. Are you ok with trying a more wide open offense that utilizes Allen's skills? If not then you let him go. Every top offensive coach available would want the job.