For a while it was fully open source. I agree that it would make more sense for it to be fully open source again and so far I've yet to hear any good reasons from @staff or others about why it isn't, see:
The only reason I can fathom is a fear that making Cloudron open source again would somehow mess up their business model due to the potential for copycats to offer the same service for cheaper.
Personally I don't see that fear as having that much risk as Cloudron has a strong community and that is something much much harder to simply copy.
But also, as you mention yourself, I think there are a bunch of people out there who don't contribute to Cloudron because it's not open source, and a whole load of people who don't subscribe because it isn't too.
A crazy scheme I just came up with:
Take a percentage of Cloudrons' sales and re-distribute back to either:
a) existing apps on Cloudron based on popualarity
b) or as a bounty for apps not already on Cloudron. For best quality, add preference for the original developers of the project to bid on the bounty and provide support.
Interesting proposal. Cloudron does already support some upstream projects with both code and money, but I'd love to see it do so systematically!
I'd also love to see more transparency about the finances, how much is coming in and going out and on what. Buffer are a really great example of this, see https://buffer.com/transparency