Source compatibility is not really helpful for people building libraries on top of your libraries though.
Since the platform is means to be “core” and “used by many libraries / projects” it should aim for binary compatibility.
It’s possible to pull off, you simply deprecate and add new APIs - we’ve been keeping binary stability in Akka since 2.3 (March 2014!), and it is a feature to be stable and remain so for libraries as core as these.