Postel's law is absolutely great if you want to make new things and get them going in a hurry, and I think it was one of the major reasons the TCP/IP stack beat the ISO model. But as you say, it's a disaster if you want to build large robust systems for the long term.
1970s was also just a different time: documentation was harder to get, it was harder to do quality implementations for protocols, people had less of an idea what may or may not work because everyone was new at this (both in terms of protocols and implementations), shipping bugfixes took a lot longer, few people were writing tests (and there wasn't a standard test suite), few people had long experience with these protocols, and general quality of software was a lot lower.
