Vax, for example, had functional clusters, versioned filesystems, and other goodies, many years ago.
Just the nature of a proprietary system...controlling everything means some functionality is easier to implement, more elegant, etc. Apple still takes advantage of this.
The hardware and OS licensing costs are high, as you suspect.
I think that's a stretch. It had file-name;rev where each time you wrote the file it bumped $rev. As an SCM guy, that's not versioning, that's automatic backups. And you have to run purge all the time to free up disk space.
I don't mean to be pedantic but this is the second time I've seen this claim in the last few days.
To me, a versioning file system has a dag, the OS creates a node everytime you close it with modifications; if two people close at the same time then just like BK or Git you fork the graph, which means the next time someone opens it it has to be merged.
As I said elsewhere, it would be super cool to have such a thing for /etc, if you wack a config, debian wacks it, you upgrade, you then get all the features that an SCM system give you for merging. I dunno what Git gives you but BitKeeper does a kickass job of automerging and has a complicated but pleasant graphical file merge.
Etckeeper [1] does this. There's also etcd, but that seems to be more akin to Puppet and Chef. Disclaimer: I've never used any of the mentioned software.
[1] https://en.m.wikipedia.org/wiki/IBM_i