Won't each endpoint have its own schema for its response?

They should, but often didn't. Today's IT folks consider microservices the reasonable default. But the logic back when XML was popular tended to be "XML APIs are very expensive to maintain. Let us save time and only maintain one."

And if you're updating things asynchronously then doesn't versioning each endpoint effectively solve all the problems?

XML schema validation meant that if anything changed on any endpoint covered by the schema, all messages would start failing. This was completely preventable, but only by an expert in the XML specification - and there were very few such experts.

But yes, splitting the endpoint into separate schema files solved many of the issues. It just did so too late to make much difference in the hatred for it.

And really, the remaining issues with the XML stack - dependency hell due to sprawling useless feature set, poor documentation, and huge security holes due to sprawling useless feature set - were still enough to put the last nail in it's coffin.