- Greater Contributions. With a source available, but proprietary license, anyone who contributes a bug fix or feature immediately loses license to their own work, or at least would have to in order for Cloudron to be able to enforce its license and copyright. Aside from the potential legal mess, this is almost certainly a deterrent to substantial outside contributions. An open source license makes Cloudron much more enticing to contribute to.
- Benefits of Broad Adoption. Those willing to do the work to run their own Cloudron instance from the source code may have been loud, but they weren't likely customers to begin with. While not paying, these potential users do offer some benefits. They're more likely to provide good bug reports, patches, and answers to community questions. Even while promoting the free use of Cloudron, they are reaching an audience you would otherwise have to pay to reach. Even if the vast majority of users were to use Cloudron for free, as I suspect the majority of Nextcloud users do, in volume it really becomes a net benefit to Cloudron.
- Long-Term Assurance. The choice to self-host one's own infrastructure can be stressful. It becomes less stressful when you know that the software your using is open source and will be viable as long as there is a community willing to keep it going. This is one reason open source users become such loud advocates. They want that thriving community to live on forever, in a way they can't necessarily ensure a company will.
- Part of a Bigger Cause. I like you @nebulon and @girish. I like what you have made, and I hope you succeed, probably more so than most companies I buy products/services from. But at the end of the day you are a company. People like companies, they support causes. It's hard for me to express how when Cloudron went from open source to proprietary it changed my feelings. I still tell people about it and have tried to make important strategic introductions. But I don't donate my time to Cloudron like I do Inkscape. I don't extol the virtues of Cloudron over all other proprietary solutions, like I do Nextcloud. Supporting a company selling a proprietary solution is just not the same as supporting a company that is part of a bigger cause. Cloudron has the potential to be part of that cause. I want it to be open source.
Addendum: This is all said with full awareness that you need and deserve to get paid. Don't listen to anyone who expects anything otherwise. For the reasons stated above, and others, I think you can still make a living and perhaps even a better living releasing software with an open source license.