Pretty sure bazzite is immutable, it is based on silverblue after all. But that's besides the point, i just wanted to point out that you probably misunderstood what he was saying. Immutable distros being mainstream kinda depends on how you look at it i suppose. Purely in terms of amount of users, with steamos and bazzite being so popular, i guess you could consider it mainstream, but how many people actually choose a distro because it is immutable? Steamos just happens to ship with the steam deck, bazzite is popular because it mimicks steamos for other devices, and if you look at something else like NixOS, it's more about the system being declarative rather than immutable. It's probably safe to say that flatpak is the most popular third party package manager though, i do agree with that.