Very true for mainstream distros, but there’s more: Linux updates in the background. No matter how long it takes(if you for example use Gentoo), there is zero downtime. And with kexec your system can be its own bootloader and can do insane stuff like starting new kernel without re-running POST, which is on servers is very important(because they have shitty BIOS that takes ages to boot).
There IS ‘lockable mode’ since System V era. It is extensively used by package managers and similar stuff.
Deleting file does not actually deletes it from disk until last program closes it. And there are ways to open file such that changes to file will not be seen in program.