If (US) employers really want people's departures to not mess things up, there's a simple tool then can use: contracts mandating notice periods. Of course, that would require consideration, so they'd have to provide that to employees that they are firing as well.

They've overwhelmingly chosen at-will employment, and they shouldn't be upset when their employees actually take them up on it. Two weeks is already an incredible courtesy when you can be fired at any time for any reason (as long as they're not stupid enough to say "yeah, we're firing you because you're black").