In that instance, having an app which centralizes the push service needs of other apps would be highly useful, as it could then be installed as a system app with the same kind of access as the FCM service. For many phones, custom ROMs are available. Average people probably won't have the skills to install custom ROMs. But the goal of OpenPush is not to replace FCM from all phones everywhere. It's to meet the needs of the subcommunity of people who want to use degooglified Android, maybe also later on integrate with non-Android mobile OSs. In that subcommunity, people likely already use custom ROMs, and among those who don't, the readiness is higher to install one.