However, I do think that it has to mean something besides “there are no other good providers of a service”. Integrations, platforms, etc make sense as being “locked in”, but not “no one else provides the service”

To me, the key would be, “if you were starting from scratch and weren’t using any service at all, would you choose a different one than what you actually currently use?”

If the answer is “I would still choose the one I am using”, then I don’t think that is locked in.