Why? What do you prefer to do instead?

why? it works and its super clever. Simple command instead some shit written in JS with docker trash

Then you are locking yourself out of a pretty much ironclad (and extremely cost-effective) way of managing such things.

Works pretty well for Nix

that's how some phone OSes update the system (by having 2 read only fs)

that's how Chrome updates itself, but without the symlink part

Nobody's saying you should deploy code with this, but symlinks are a very common filesystem locking method.