Well, if you like Arch and NixOS the most, I think you’re a tinkerer/ someone, who likes his OS in one definitive way.
That’s totally fine, I love that!
But Silverblue is, I think, more catered towards people who love reliability and prefer it over customization.
I, for example, find SB pretty much perfect how it is.
… well, actually, not 100%. I use uBlue (main), which is basically a modified image of the Vanilla SB with some minor QOL-tweaks, like a few exchanged apps, automatic updates, and so on.
You can also create your own images with this project, with effects similar to Nix.
But if you want your own, individual, config, Nix is just better.
I’m really excited what immutable distros will bring in the future. I follow them (Nix, SB, VanillaOS, etc.) closely and think, that immutability will be the future of Linux, even if that’s a trope.
We already containerized everything and never touched the base OS on servers too for decades now, why not on desktop too?
That’s the main advantage of Linux, and we should use that.