Most firings I've seen have much more to do with the people involved (harrasment, incompetence, interpersonal problems) than with wild changes in the company's trajectory, though I will admit that at early-stage companies this could be reversed since things are much more uncertain.

You misread.

You're being asked to imagine yourself as the executive - you're the person with firing authority.

A manager who works for you asks you to fire someone on their team. There are a hundred people on various teams in your group, you don't have first-hand knowledge of all of their performance. You want to have a system in place for making sure this is a fair request, vs "we argued a few times the last week" or, in the really bad sorts of cases, something like "we had an affair and it ended badly and I'm trying to clean up the mess by firing them."

You're totally misunderstanding the situation.There are three hypothetical people involved here. An employee, who works below a manager, who works below a VP.

The manager wants to promote the employee, but she needs permission from the VP. So the manager asks the VP to promote the employee.