Nothing about UML specifies its use in relation to when or how you wrote the code. Like I said, it's a false dichotomy to suggest that we can have source code or UML but not both. Yes, UML diagrams or any other design docs rot quickly. But like you said, we can generate diagrams after the fact too (or in lieu of ever writing UML diagrams before coding).

> perfectly good way of representing the details of a system - it's called source code. ...

> simply use a tool to generate a class diagram from the code

Once you resolve to generate that class diagram you have a choice in how to represent it. UML is just one way, proposed as a standard way, to represent that visually.