Very interesting conversation.
Getting upstream projects to support Cloudron distribution as well as others (eg. Docker Compose, Helm, Flatpak, etc.) would be the most scalable, but probably a hard sell. Additionally, even if it were done, would that be the right call? Even projects like Debian maintain their own repositories.
I'm not sure if this is available somewhere, but if there were install metrics for apps, that could go a long way in showing upstream projects how many users are using their application on the platform and encouraging them to offer more support, even if Cloudron maintains the repo.
For example, this is how Home Assistant reports install metrics: https://analytics.home-assistant.io/#integrations
Actually, Home Assistant also has their own "Addons" features which installs managed Docker based applications from a repo that is mostly maintained by the project leads but is also community supported. Much like Cloudron. They also have addon stats here: https://analytics.home-assistant.io/#add-ons