You say that, but once Tom Brady left New England fell apart under Belichick. They weren't one franchise QB away from being great by the time he left.
Being an NFL head coach is an incredibly difficult and demanding job. These guys basically have no life outside of football and it's why so many of them end up divorced. And the burn out rate is really high. Belichick for example, as great as he was, really just looks like he is done and burned out.
And frankly Gruden was never a great coach. He caught fire for a couple of years but the league caught up to him and he left the game for 10 years because he was burn out. And frankly, when he came back he wasn't a good coach. Again, he had a 22-31 record in his second stint with the Raiders. There is no reason to believe he is going to improve on that.
If there was a HC candidate out there with experience that had not proven to not be a good HC by getting fired, then sure hire him. But at this point they need to look for new blood that will bring new ideas. I'd consider Ben Johnson, Slowik, Aaron Glenn, and Rizzi off the top of my head. And I'm sure it wouldn't take long to find several other very qualified candidates for the job.