I agree. A schema language that can be used to codegen has to be very rich, and uses of it have to be very detailed. UML mostly only found a niche in writing

I agree. A schema language that can be used to codegen has to be very rich, and uses of it have to be very detailed. UML mostly only found a niche in writing docs, in which case it would be better to generate the UML from sources, which then requires having access to ASTs or alike. It's no wonder that UML is mostly hand-coded and only for docs, in which case UML is a bit heavy-duty.