Bazzite is not immutable, and SteamOS is as mainstream as it gets while being A/B root immutable.
All of them ship Flathub because it’s ready for public consumption.
If the attempt here is to argue that cloud native isn’t mainstream and change topics from flathub, you are proudly in a bubble of 3% of the computing industry while your peers in the Linux server space and Android run circles around you.
Yes, we are. It’s exactly why it shouldn’t be done and why Fedora is the only project wasting their time with this.