You already have the option to install apps from others in a general sense because of Docker.
I pay for Cloudron not just because it is Docker-based, but because of how easy the devs and admins and owners have made it to use. I think adding unsupported apps to the mix even more will distract from the appeal of Cloudron. The extra apps that are available are there because the app is in the pipeline to being added to the official Cloudron store; they have been/are being worked on by people who like the Cloudron approach and want to add to that, and not to a Docker-experience in general.
The price jump from no-support to support with nethserver is a huge jump from 48€ to 250€! I think Cloudron has a good pricing structure because they have limited what they have to support. To address one of your first points, I left Yunohost exactly because of how many low level quality apps that they have, and all the hassles and problems that approach introduced. I wouldn't want to see Cloudron go that route. I like the line being thick! I am already receiving all that I hope for from Cloudron.
All that being said, I think we already have a pretty good 'community repository' of helpful users and super helpful admins. For anything that I want to do that is beyond what Cloudron offers, I just tinker away on my other VPS.