>are almost completely arbitrary? What constitutes a "senior", "lead", "principal" and "staff" X, respectively, has so much overlap that it really depends on the organisation

Titles at least useful to understand the hierarchy, but roles truly mean nothing. Sometimes the adult in the room is a PO, sometimes it's EM, sometimes they are responsible for the timelines and "project stuff", sometimes it's a Senior Engineer. In some places a QA is effectively doing PO stuff.