We have a machine at work which is beefy and works as a server and backups for many many years on windows 10. Why the hell should I upgrade my buisness critical system ??

Because you should be using a server grade os instead of janking things together with desktop OS installs that just make everything so much harder (and aren’t supported for as long).

Sorry, I have to clean up installs like this at least once a year when we take on clients from internal IT that just made things work instead of making something that works right, so I’ve got opinions.