The question is why is the employee failing? Is he in a situation that is hard to succeed or does he not have the tools for success? It is always a somewhat arbitrary call in these situations. Why was Belichick a failure at Cleveland? With more time would he have succeeded? This all gets down to a lot of opinion and conjecture (both of which are in great supply on All Buffs).
I don't disagree about needing to understand why a person is not performing where you want but I still maintain that "we can't find anybody better" is not a reason to keep somebody.