Weigh up the cost of the upgrade against the benefits; estimate, include the non-specific benefits such as developer-satisfaction (why would I work on .NET 1.0 nowadays when I can get plenty of jobs working in 4.0?), but make it reasonable.
If the benefits outweigh the costs short- and long-term then your argument is made. If the benefits don't outweigh the costs short- or long-term then don't do the upgrade.
More often than not, the benefits do outweigh the costs in the long-term but not in the short-term. In those cases, you need to present the case to those who make such decisions and let them decide. And if they plan to go with the short-term answer, that's OK.
If we did everything thinking only of the long-term, we'd get nothing done in the short-term.
Next time there is an upgrade, do the same thing. The values will be higher and the upgrade more justifiable. But remember that it's still a decision above your head.