> You don't need to explicitly install your replacement code and start that server

This is a double-edged sword though. Let's say I have a PHP file that "include"s another one and a request arrives in the middle of such a deployment where the first PHP file has been replaced but the second has yet to be - you now have undefined behavior.

Filesystem-based URL routing is one of the "features" I hate the most in PHP as it leads to countless security issues - simply uploading a file in the wrong place (or sometimes merely with a wrong extension) immediately turns into an RCE.