For the record, access to push notification APIs only needs to be done by the developer of the app the users are using. Small servers never need to communicate

For the record, access to push notification APIs only needs to be done by the developer of the app the users are using. Small servers never need to communicate with Apple/Google themselves - any necessary notifications are routed through a gateway operated by the app's developer (which is also necessary because only the app developer is given the API keys and permissions to send push notifications to their app). At the time of writing this, 90% of 363 tested XMPP domains support push notifications. I'm not disagreeing with your overall point, mind. There is lots to compete with, and due to the lack of a good business case for open networks, most XMPP projects are by open-source volunteers and the majority of public XMPP services are similarly operated by volunteers. There do exist projects with funding though, and there are multiple companies actively engaged in paid XMPP projects and deployments. But these all tend to happen behind closed doors, they're typically not deployments for the general public, or the XMPP is quietly under the hood of a larger service/product (such as happened in Zoom, WhatsApp, and many online games such as EVE and Fortnite).