Well disk (or tape) images are as old as 'dd' and 'tar', that's not the revolutionary part. If the disk is writeable the state is still constantly mutating, so you're fighting a war of attrition (it's configuration management at that point, which is terrible). But a read-only disk that doesn't accrue changes, and only needs a reboot to fix, that's the revolutionary part. Anybody who ran thin-terminals can tell you how reliable and easier to manage those are than a full-blown OS.

At some point in the future people are going to realize that every system should work that way.