This seems more philosophical than on technicalities. If this is correct, would you mind elaborating on the philosophical side?
They are not well developed now
Even if this were the case, shouldn’t the constant development and continuous improvement result in something that’s (eventually) well-developed? The only way I could see this holding some truth is if by design the ‘immutable’ model (whatever that is) happens to be broken or something like that. Like, how some file systems are simply better than Btrfs (or any CoW filesystem for that matter) for specific tasks; i.e. ensure to use the right tool for the right task. So, do you pose that ‘immutable distros’ are by design not well-suited? If so, why?
so it’s a bubble that should pop soon after people realize they are not ready yet
So you (actually) acknowledge and imply that it will become ready at some point. Or not? Furthermore, like how do you reconcile this with Fedora’s ambitions for Fedora Atomic? Or how NixOS is going strong (perhaps stronger than ever) while it’s been in the making since before Ubuntu?
and have a lot of disadvantages.
And advantages*. Or do you ignore those?
Also they are unsuitable for old PCs
This is false. What makes you think that?
and Nix seems relatively good for them
What’s “them” in this sentence? The “old PCs” you had just mentioned? Or something else? Furthermore, if it is the “old PCs”, doesn’t this directly contradict with “they are unsuitable for old PCs”?