Thank you, Luca, for this post.
I agree with your position.
The MoodleMootDACH Matrix space has been around for several years now and is even the primary vital place for discussions in between the conferences.
Last year in Lübeck, the Whatsapp Channel was, as far as I had understood it, set up as an _additional_ channel (in addition to Matrix) and as an experiment. I cannot tell how many people really engaged in the Whatsapp channel last year, but I think I can tell that there were several people (including me) who felt cut off from official announcements as they were not posted anymore on Matrix at all.
My plea is just as Luca's:
Let's continue to use Matrix on the conference in the sense of 'eat your own dog food' and keep the Open Source banner high instead of selling our soul to Meta just because 'everyone is using it' and 'it's so much simpler'. Someone could even pitch a session 'how to install Matrix on your mobile phone' in the very first time slot.
Just my two cents,
Alex