Yeah, Silverblue is nice; not my main distro, though. I came from many years of Gentoo on almost all machines I had. Debian stable is currently the most practical for me.
The immutable variant of Fedora is nice for the low maintenance computers, although I needed to get my head around podman to not overuse rpm-ostree. So far so good. I think it will not replace the distro on my main machine, but I see the way it works as a general potential future. An immutable variant of Debian stable would be something I'd consider switching for.
I delete older ones as soon I downloaded a new one. I guess, my isos take well below ~15 GB of space. It is only Debian, Fedora and Gentoo afterall.