It is more complicated than that

* losing the low performer in this context means you lose a head count. If you keep the low performer during hard times, you can hope to get them backfilled when the economic environment gets better * there are often more work to do than you have resources for: if as an EM, you have to deal with PM who refuse to adapt to capacity, you can simply assign the low perfomers to the projects you don't care about. * etc.

once you reach a certain size, those issues happen in pretty much any organization in my experience.

Even if a low performer reduces team productivity, having more team members rather than fewer might be in the manager's personal interest.

Agreed. It's easy to plan around and measure the potential business impact of no one. It's challenging to do the same with a low performing or otherwise inconsistent employee.