[happytoexplain]

>just a glue for the user.

This is fairly contradictory. The reason gluing APIs together can be easy is because APIs are (usually) documented, mostly deterministic, have a limited set of operations, are designed to be glued to, and various other complexity-mitigating characteristics that don't apply to humans. "Gluing" with a human is intrinsically complex, so the way we use the word "glue" doesn't really apply.

[seba_dos1]

Some UIs absolutely are simple enough to be considered glues for underlying backend.