If you can a find a way of A) getting live scrobbles from Last.fm into Rocksky without having to change the clients that are all using Last.fm already and/or B) get historical scrobbles out of Last.fm into Rocksky, I'd think you could potentially take over the ecosystem relatively quickly :)

Yes, interoperability is super important to me. I don't want users to rebuild their entire setup just to try Rocksky.