This has been discussed before in #1377628-7: taxonomy_get_term_by_name() should be taxonomy_term_load_multiple_by_name() ff, but deferred to a followup which finally didn't get filed.
"load_parents_all" is bulky, unintuitive and easy to confuse. "load_descendants" would be much clearer.
There's also been some discussion in #251595: Add taxonomy_term_load_descendents(). However this rename would make sense, whether that one lands or not.
Note that it's also debatable whether the function should include the actual term itself. In the end, a term isn't its own parent, nor descendant. We could add a switch parameter "include_self" to keep it convenient while documenting that oddity. However that might be out of scope here.