> But that doesn't mean they "had full awareness of every language feature available in the alternatives", and even if they did, smart people are still susceptible to the blub paradox.

I'm pretty sure Ken Thompson &co were fully aware of generics and sum types (to use your own example). It trades the implicit complication of implementing and using those (and other) features for strong concurrency. For a lot of people, that's a good tradeoff to make.