Well except most useful software (not all) interacts with other systems. I loved my old mail client until it stopped working with the latest encryption schemes. A nicer way around this is like what plan9 has, where programs don't worry about handling secure connections etc, so you don't need to patch every program.

some people still use a 90's version of wordperfect, and that's fine, but if you have software that depends on other software - example Qt3 was dropped everywhere, even though there's plenty of useful Qt3 software around, it needs to be upgraded.

So even if your system wasn't buggy, it may be incompatible or the dependencies no longer are updated (like with Qt3, which had networking and other helper functions), you wouldn't want to keep using it.