With more time, testing, and thought on the subject of immutable distros, my personal take is that I don’t see them becoming broadly popular. Especially on the desktop. The benefits make sense for some servers, on kiosks, and in tightly controlled environments, but for everyday Linux users and admins, the trade-offs are too significant.
Immutable distros significantly change the basic model that Linux users are used to. Traditional distros let us install a package with a command, tweak config files in place, and rebuild or fix things on the fly. Immutable systems explicitly discourage that.
Normal package managers (apt, dnf, etc.) aren’t used because the root is read-only, and installing software often means relying on Flatpak or container-based tooling. That, to me, introduces more complexity than it actually adds stability.
I suspect immutable distros will continue to grow in specific niches only. For general use,it seems like an unnecessary complication for an already stable and secure OS that is Linux. 