As an engineer once said, "if it is not broke, don't fix it." In the case of an OS, however, how do you know if it is broke?
If the computer is very versatile and connects to any network or network of networks (ie. the Internet) then the machine should be maintained in a stable, current condition.
If the computer runs in an air-gap and security is not an issue, then do like the engineer above said and don't fis it.