TBF the unix model originally was read only mounted / and /usr - typically in a separate partition - so they were immutable for decades. It's only later when home users started using single partition models that that really broke.

But the separation is built right in.. /etc for config, /home for user stuff, etc. so there's really no need for another layer.. it's not like windows were it's common to store the configuration in the same directory as the binaries.. so not really seeing what the current fad is about. Presumably something will come out of it, maybe even a standard..