Most tech companies I've worked for in the past years have had about 30-40% new people every year through a combination of natural turnover and growth.

You're right that if a company never grows and nobody ever leaves yet they still try to isolate 6% to fire every year, that's a bad practice.

However, nobody gets hiring perfectly right. When you're hiring hundreds of people every year into a department, you're going to get a couple people who don't work out.

I think people on HN get nervous around these discussions because they imagine companies as static collections of individuals, but at a high growth company you actually have a lot of new people coming through all the time.

It's also bad when companies never fire anyone. If you just let everyone stay until they quit, you accumulate a lot of people who aren't working out while their high performing coworkers get frustrated and leave. I've seen this situation too and it's not good.