Freudian crap coming up, but here's my take. Fragile and stubborn egos often come from insecurities. Sancho plays like someone who's lost belief in his ability - like he's already lost the 50-50 or the tackle or the sprint to a through ball or an attempted take on even before he's stepped onto the pitch. He's insecure about his ability because he hasn't delivered in so long, he doesn't know if he can. And he'd do anything but admit it because it's a difficult pill to swallow. He'll avoid committing to the cause, enjoy his fabulous wages, find faults in everyone but himself. He'll blame the manager for not picking him, complain about ill-treatment compared to teammates, and make public posts and burn bridges in this fight because all of that is still better to him, in his mind, that stepping onto the pitch giving it his all and realizing it's just not enough anymore. He's so afraid of failing at this stage that he'll do anything to not try.
The solution was to intervene earlier. An arm around the shoulder, and stubborn persistence from the manager about starting him and providing positive reinforcement even when things aren't going his way. The solution is regular time to get his confidence up through moments where he ends up surprising himself with how good his own play is, and sustaining it through validation from teammates and manager. But all of that that requires patience and a tolerance for poor returns that a club like United cannot afford to provide. So I think he's done here. If he finds a club where he can recover his confidence and overcome his insecurities, there's still a baller in there. But it won't happen at United, and it won't happen if he keeps trying to run away from his fear of not being good enough anymore. The latter will be dicey given the sycophantic poor company such insecure high earners keep.