I don't believe I mentioned the number of people, merely that upgrading when nobody was using the system creates another risk that you won't know about till much later.
People in IT seem to want the "perfect" solution, which doesn't exist, or at the very least a black/white kind of thinking. Everything is tradeoffs and it's important to understand what those tradeoffs are. I've also seen people seem to think all situations and organizations are the same. (Obviously very, very wrong).
But I will say this. In some cases the best solution might be to upgrade the system when people are still using it that it can be switched back quickly.