I beg to differ. I think he has been a monster and one of the integral performers for your 2020 treble, and thereby actually somewhat underrated. The only thing overrated probably was the contract Bayern gave him, but that was down to the precedents set with Hernandez and Sané.
Yeah he struggled with form and fitness but then he has shown absolute top mentality when not being first choice anymore as a squad player: your club board tried to bully him out and basically sold him off publicly because Tuchel and Nagelsmann didn't like his profile (the expert that is Tuchel of course insisting on the mythical "proper DM" from the Premier League in Palhinha) and the club wanted to save wages, but he just kept his head down and, despite Palhina being bought, Pavlovic emerging, and everyone writing him off, kept showing up for the team when needed. And now he's back to being an important performer, even if not first choice ahead of an excellent Pavlovic.
So I'd bet his teammates and also the match going fans wouldn't share your assessment. Actually a bit of a similar story to Gnabry.
Should he get the kind of wages he's currently getting and a long contract? Of course not. Should he be regarded as one for the future, obviously not. He's on a free, he's both physical and technical, can play anywhere in midfield and even slot in at CB, has vast experience playing at the top, a goal threat and a bullying presence. I also think he'd prefer having a go with a big PL club on a shorter, less lucrative contract to getting a comfy pensioner deal in a lesser league.
He'd be a perfect Casemiro replacement for a couple of years, while we properly invest in young top players alongside him.