> If a person is performing badly, document that and then fire them.

Which is entirely subjective. There are a billion criteria you can reasonably evaluate someone on, so it's trivial to make the average person look below average by choosing the things they're bad at as the things to evaluate them on.

A company will always be able to get rid of an employee they don't like one way or another. The only choice you have is how much bureaucracy to impose. But it's the same amount of bureaucracy even when the company has a justifiable reason.

For you to be so dismissive of the breadth and depth of documentation needed to withstand a potential lawsuit, I suspect your experience is limited to the employee side of the relationship.