The point is that you're probably going to have to mess with it eventually, and the amount of work that takes goes up the longer you put it off. If you're talk

The point is that you're probably going to have to mess with it eventually, and the amount of work that takes goes up the longer you put it off. If you're talking a completely offline system that's basically a utility and doesn't even need security patches, then how many years of support the OS provides becomes irrelevant, because you theoretically don't need those patches anyway. But if you're concerned with staying "technically up-to-date," as in "the vendor says it's supported even though it's X years old," then there will almost certainly come a time when you reach the end of the line and have to reconcile that with everything you didn't keep up with. (It might be after you're out of there, though.)