It seems pretty reasonable to say, “if you want to send a push notification from an app, you should be the controller of the app”.
The alternative would be that anyone could send notifications to an app, which seems destined for abuse. There could be a system where apps could have some mechanism for allowing authorized third parties to send push notifications, but that seems essentially the same as routing those through a central system.
> The alternative would be that anyone could send notifications to an app
No, the alternative is that anyone with a password the app generates can send notifications to any app, which is (effectively) how WebPush and UnifiedPush work. The server that knows the secret URL can send a notification to that website or app.
The alternative would be that anyone could send notifications to an app, which seems destined for abuse. There could be a system where apps could have some mechanism for allowing authorized third parties to send push notifications, but that seems essentially the same as routing those through a central system.