APIs defined in XMLRPC, CORBA, SOAP, et al, can have clients and servers implemented by anybody.

Programmers don't seem to learn from history and struggle with thinking over time. These formats worked well in a time when a single party (or second party) controls the server and client, when services were very consolidated. Now that the web is becoming more and more centralized and closed, it follows that RPC is making a comeback: widespread interoperability is not much of a concern.