riddlemethis As the community manager for the project, along with having been a community moderator for a very long time now, let me give you my perspective:
When I first started engaging with the GrapheneOS community a number of years ago, I created a Matrix account just to be able to speak in the GrapheneOS rooms. This is not an uncommon occurrence in our community, and I would say that if we were to take a poll, the percentage of people who made a Matrix account just for GrapheneOS and don't use it for anything else would be surprising.
I can understand the "principled" position, but there are many other factors to consider if you want to create a solid community. Matrix has horrid moderation tools, critical state-breaking issues that have led to room bricks, etc. This has greatly impacted our community through having to re-create the rooms after a brick many times, along with long-time helpful community members deciding to leave due to being exposed to graphic materials by people attacking the project because all we could do is remove it as soon as possible; the lack of moderation tools didn't give us any serious proactive options.
Matrix was chosen because that's where the community already was after IRC stopped being the main platform for the project. It's still offered as an option to this day, as is IRC, Telegram, and more recently, Discord.
Discord in a few months has grown immensely. It's a space where members can be shielded from the attacks, and also lowers friction as a great number of people already have pre-existing and long-established Discord accounts, which is not going to realistically be the case for Matrix or XMPP.
Adding more options on top of Matrix wouldn't make sense if the options that were added did not address the problems that we were dealing with on Matrix in the first place. Matrix remains an option, but by also offering Discord, we're offering people the chance to use something familiar, lowering the bar to entry, along with something that allows us to uphold the standards of protecting our community that we want.
GrapheneOS is useful to people when they get to use it. Let's not make the community surrounding it obscure. Extending a welcome to people on a platform that's not only better in terms of being resistant to attacks but also familiar is a good thing, not a bad thing.
As things stand, we would prefer to not have to host something else, and even if we had the capacity to host something else, it would have to be something that would solve issues instead of just creating different problems without a clear benefit.
I hope the above provides some context behind the thought process.