Though only for employers that don't care who they hire in the first place, if you fire someone simply because they might be harder to fire lately you don't really care about who you hired.

I actually came to the opposite conclusion: you really care about who you hired, because you define who they work with. If you hire a low performer or someone that isn't a good culture fit, the productivity of your other team members will suffer.

The claim is you would fire someone purely because they are about to age into a bit more job security.

If you fire some because they are a bad employee in some well defined way, that's a completely different situation.