NAT wasn’t started as a way to make the address space last longer. A few decades ago you could get hundreds of thousands of IP’s by filling out a form with ARIN without any serious justification if you wanted them. I worked at an ISP and IP’s weren’t a scarce resource.

NAT started because having a network didn’t mean you were necessarily participating in the Internet. Globally unique addresses weren’t that important. At some point you had this decentralized situation where local networks wanted to bridge their users address space to the Internet without renumbering everything and thus NAT was born.