I started using Unix (as a sysadmin and systems programmer) in about 1986, and my recollection is closer to Rob's.
1. Never used tar(1) for backups till much later, always dump(1),
2. Never wanted to separate backups or upgrades of / and /usr, but always wanted and needed to do that for /usr/local and wherever we put home dirs in those days (which I don't recall right now).
3. The / and /usr thing was definitely explained to me in terms of disk space, more by comparison with the VMS system I was also managing which had 1MB drives. But I was too late to have "been there".
It's entirely possible that there were multiple rationales/retcons at various stages, maybe in serial or parallel; we could all be right. My 'been there' is also from ~1986 (AT&T 3b2, 300 baud acoustically coupled to an AAA-30). That said, the idea of / immutability and /usr variability was highly valued for its systems encapsulation properties for many decades, regardless of how we got there. As others in this thread have indicated, it's still considered valuable for that reason in space-constrained and security-critical domains (e.g. IOT).
1. Never used tar(1) for backups till much later, always dump(1),
2. Never wanted to separate backups or upgrades of / and /usr, but always wanted and needed to do that for /usr/local and wherever we put home dirs in those days (which I don't recall right now).
3. The / and /usr thing was definitely explained to me in terms of disk space, more by comparison with the VMS system I was also managing which had 1MB drives. But I was too late to have "been there".