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).