Being on top of pingbacks makes no sense, xml-rpc is dead and a bad idea in a first place, also a lot harder to implement than just reacting to a ping + pretty much incompatible with the pull concept. If they started there, they probably did not know trackbacks existed.<p>Webmentions are no simpler than trackbacks, and to have them diverge for no reason makes everything more complicated for the engine. It is no straight-forward simplification at all, thus no good thing.