Calling it a "brittle hack" implies that I don't have any control over the other endpoint. In reality my employer controls the endpoint I'm hitting. So it's not

Calling it a "brittle hack" implies that I don't have any control over the other endpoint. In reality my employer controls the endpoint I'm hitting. So it's not a brittle hack in our organization even though it would be a brittle hack if we were, say, hitting a Twitter API. In my opinion it's the same kind of optimization as massaging your data inputs to span cache lines in your MCU to improve performance. By requiring the data to be well-specified in its format you can ignore all of the clues that XML provides about the format of the data, because you can assume the format in advance. So all the stuff like schemas and validation become completely unnecessary because you already know what valid data looks like.