Can't say I think it's a bad decision for United. Amorim was never gonna do anything there, the 3-4-3 obsession was pure embarrassment. And this wasn't the first time he'd flirted with trying to get himself paid out this season.
They'll have huge trouble getting anyone with the ability and temperament to fix their issues but if I were a United fan I'd see no harm in getting on with it versus sticking with a resentful coach achieving crap results.