The optimist in me wants to claim that not assigning any range for local networks would have lead to us running out of IPv4 addresses in the late 90s, leading to the rapid adoption of IPv6, along with some minor benefits (merging two private networks would be trivial, much fewer NATs in the world leading to better IP based security and P2P connectivity).
The realists in me expects that everyone would have used one of the ~13 /8 blocks assigned to the DoD