There are a few things here. From a technical level, how do you think GCM/FCM is implemented? A socket is hardly a scarce resource and keeping 2 open is only 1/65535 more "expensive" (from a kernel standpoint not power/energy) than multiplexing two streams on one connection. It's possible FCM uses some micro polling strategy so that the radio can sleep for short bursts, but I don't see why the same mechanism can't be exposed to other apps if that was the case.
Second, from a product level there is a double standard now. Apps that build infrastructure using Google's platform services don't have to put a persistent notification in front of users and clutter the phone ui. Apps that choose not to depend on google (the "open source" as it pertains here but you're right it's not limited to open source apps) do. That's a pretty obvious power play.
If you want to see all the stuff going on in the background why are things that buddy up with google exempt from that in your eyes? I could flood your phone with a high priority FCM message every 500ms effectively making my app run all the time and you wouldn't know. There will be apps that do it (as there are with APNS on iOS) and it is in fact far worse for battery life. Maybe we differ on this point but there are certainly things I want happening in the background that don't need a ui. That's the precedent on the desktop anyway.
BTW The ordering of FCM messages is not guaranteed so they're not even as useful as a TCP connection.
Second, from a product level there is a double standard now. Apps that build infrastructure using Google's platform services don't have to put a persistent notification in front of users and clutter the phone ui. Apps that choose not to depend on google (the "open source" as it pertains here but you're right it's not limited to open source apps) do. That's a pretty obvious power play.
If you want to see all the stuff going on in the background why are things that buddy up with google exempt from that in your eyes? I could flood your phone with a high priority FCM message every 500ms effectively making my app run all the time and you wouldn't know. There will be apps that do it (as there are with APNS on iOS) and it is in fact far worse for battery life. Maybe we differ on this point but there are certainly things I want happening in the background that don't need a ui. That's the precedent on the desktop anyway.
BTW The ordering of FCM messages is not guaranteed so they're not even as useful as a TCP connection.