I'm not knowledgeable enough -- what would it take to escape the Apple/Google duopoly?
I'm imagining a future where you buy a smartphone and when you do the first configuration, it asks you which services provider you want to use. Google and Apple are probably at the top of the list, but at the bottom there is "custom..." where you can specify the IP or host.domain of your own self-hosted setup.
Then, when you download an app, the app informs the app provider of this configuration and so your notifications (messenger, social media, games, banking, whatever) get delivered to that services provider and your phone gets them from there accordingly.
Is there anything like that in the world today?