The store I'm suggesting would contain commercial, non FOSS apps as well. There are some apps I use that will probably never be open source and there's no website version either.
For wider adoption, I think this would be necessary. Otherwise these phones will always remain a niche product.
FlatHub has commercial, proprietary apps in Flatpak sandbox format. It's generally targeted at "desktop" users, but the Flatpak sandbox ought to work just as easily on mobile devices based on mainline Linux, including the Purism phone.
For wider adoption, I think this would be necessary. Otherwise these phones will always remain a niche product.