If you want to guard against persistent malware, you don't want your system partition to be writable. You also additionally need a chain of trust in the boot process to get to this (trusted and signed) immutable file system. That still leaves the issue of user-downloaded apps/code and how to run them securely in a sandboxed manner. Which Android does reasonably well, but it is perhaps too restrictive for server/general use cases which silverblue is trying to address.

About the configuration issue, if configurations are transactional, you get transactional properties. And that would be quite significant. If you hose your system with an apt/yum update, you don't have much recourse unless you also took a filesystem snapshot before it (which you can do with zfs/btrfs/lvm-thin etc. and people shoehorn these things for precisely this reason). They are all different means to approximate the same end which is transactional package management.

System files aren't writable by normal users in any Linux distribution.

System recovery from backups is pretty easy and well understood too, so I'm not sure what benefit this would bring.

If you want transactions you can install on btrfs and use apt-btrfs-snapshot to automatically take snapshots. It seems this isn't that well-known though, probably because the problem it solves isn't very serious.

As for Android, it's already a frustrating system on phones, something like that on desktops would be total trash.