But if you don't deal with the root cause, FSG, changing the manager will only have limited success. I'm not saying there isn't better than him out there or that no one will do better, but if nothing else changes then we'll fail again.
It depends on what you mean by fail. If we're talking winning the league, then yes I suspect no manager can be expected to win the league all other things being equal and would therefore fail.
However, would you consider finishing in the top 4 a failure? If not, and if there are better coaches out there as you have acknowledged there might be, then I don't see how you can conclude they will certainly fail?