There's a real danger of keeping an inadequate manager on for too long. Good players get perceived as deadwood, and eventually another "rebuild" has to happen before the new manager can be evaluated on anything.
This has already started happening, which is why I was adamant we should have replaced Ole when our stock was still high last season. For instance Pogba's already been given up on. And even if I say fair enough let him be sold as deadwood, the same thing seems to be happening to Martial right now who was our POTY last year. And it's guaranteed that they won't be the last; the only players somewhat exempt are the relatively new players, and even amongst those someone like Donny is being misused.
A good manager would make these players perform at least level with their valuations, so even if we sell them it won't be as "deadwood", at very reduced prices. We're damaging our assets the longer we hold on to a manager that can't elevate them, and if this keeps going on there will soon be talk of yet another very expensive rebuild.