It'll still be there, but the symlink will go from your path. The nice thing is that if you open another shell and use it again, it's already there, and the symlink is reestablished. If you want it installed globally then it's still there and it just gets added to your path.

If you start to run our of disk space, then a quick garbage collection will remove all unused packages and builds.

The other nice thing about the nix-shell is I can incorporate pip into it (and probably other package managers as well, though I rarely use anything other than Python). So I can open a shell, add various lib type dependencies and any Python packages I want. All of it is only available in the shell, and when I exit they're gone as far as my OS is concerned.

You could clone a repo that contained my default.nix file, jump into a shell and be working with and identical environment to mine.

I have no idea why everyone isn't using NixOS.