I actually see each distribution’s package management system as the primary app source, not Flathub - but I guess your experience and perhaps preference will depend a lot on which distribution you use. I see the point of your example (being stuck on an old Ubuntu Linux and wanting an up to date Firefox), but using a rolling release distribution also solves that problem (in a different way). Anecdotally rolling release distributions seem to be gaining popularity.
I’d encourage you to read The modern packager’s security nightmare. It’s written by a Gentoo packager and mostly deals with static linking and bundling dependencies, but I think some of the arguments are interesting and transferable to the Flathub discussion.
Overall, getting to shorter QA cycles and being able to test applications you can’t live without in such a way that you can find ways to work with a rolling release distro seems like a better end state than having to worry about which of all your software have patched a bundled library that’s got a critical vulnerability. This seems worthwhile regardless of Linux specifics (like where the app comes from or who packages it), given the security landscape and more urgent patching timelines.
I thought it was interesting that you didn’t talk about QT - you can actually use it to build non-Electron applications for various desktop and mobile OSes including iOS and Android, and obviously SFOS uses Qt under the hood as well. Fun fact: there’s an old version of desktop Wireshark in OpenRepos, although I don’t know for sure if the screenshot there is from the QT or GTK version (the experience of using that of course won’t be as good as something built “the proper way” for SFOS).
Re #10, security for banking etc, you may find it interesting to know that e.g. the German eID app is open source and someone’s even made a port for SFOS. Many banks’ internet banking portals work just fine with e.g. an external card-reading device that generates codes.
Re: developers: I think it’s worth pondering the consequences of some of the things you’re suggesting (e.g. Linux platform consolidation). If they bring all the users, but alienate the developers, that will lead to problems.
PS:
[…] excluding those requiring professional software, for whom Linux isn’t an option altogether
I think there are a fair number of professional software users using Linux, see e.g. Survey Shows 60% Of VFX Designers Are Using Linux (2021)…