Discarding the old server and replacing it with a new one is a very brute force way of dealing with the problem. That's not to say it's bad (it will obviously work exactly as advertised), but I don't see it working well for me. In particular, I want something with a quick turnaround.

I very frequently deploy to a virtualbox VM during development. With NixOS this often takes <10 seconds, and that still feels slow. I cannot imagine that you can do immutable deployment anywhere near as quickly or conveniently (but I'd be excited if you can tell me I'm wrong).