XCB did not largely replace Xlib. In fact, some (all?) implementations of Xlib are built on top of XCB.

Maybe the technical politics have changed, but I feel like I remember there was some push in the late 2000s to rewrite libraries that were using Xlib to instead use XCB.

Regardless, that's sort of my point: having a lower level fiddly layer is a desirable quality, and Xlib being rebased on top of it isn't exactly a counterexample.

Turns out you want to build higher-level interfaces on top of lower-level interfaces, not the other way around.