To be fair, every modeling tool had terrible UX at that time - save for some domain-specific ones (e.g., NI had a great tool, but it only worked with their hardware components).

If those advocates were sellers, then they were all proposing this as a panacea - and no tool is a panacea. Anybody who promises to solve all your problems is selling snake oil.

Which is also a reason your are seeing more textual front-ends for UML.

Actually, ERD are not specifically supported by base UML, but an approximation can be done using class diagraams and, with a little specialization (profile) the rest of ERD can be represented.

I would say it really depends... There are UML diagram generators with whcih you might want to integrate (e.g., PlantUML) that would meet the needs of someone creating documentation. Alternately, they may use their own UML tool (or a free one like Eclipse Papyrus) to create the diagrams they need and then extract those as pictures that you can import.

IMHO, adding full UML support is crazy for your application and would potentially add years of development.

Even adding simple, non-model based UML creation would be a significant burden if you are a small company. Plus, there might be open source generators or generators you could call programatically to generate the images desired.

But first, you probably need to better understand your client's actual needs. I have doubts that they would need all the UML diagrams...