About scalability, a possible compromise might be making some distance-based limitations. I.e. limit subscriptions per prefix or adjust packet drop rates. That

About scalability, a possible compromise might be making some distance-based limitations. I.e. limit subscriptions per prefix or adjust packet drop rates. That way regional SSM can still be useful and local islands (which whould hopefully grow over time) could be bridged by unicast links or - if they have overlap - by leapfrogging. Even at the ISP level there would be still be use-cases for SSM.