[xyzzy_plugh]

Have you looked for any evidence? There is plenty.

Sum types have been discussed since before the initial open source release of the language, at least according to some of the issue threads such as https://github.com/golang/go/issues/19412

If they're laughably simple, then please contribute a proposal for how to add them to the language. You'll find no one is really fighting against the concept of sum types.

[TwentyPosts]

They're 'laughably simple' on a conceptual level. Adding them to the language in retrospect might genuinely be a bad idea at this point.

The thing which I am personally salty about is that they weren't there to begin with, and that the language wasn't built with them in mind.

[xyzzy_plugh]

Go development began circa 2007, I do not believe sum types were common place nor do I believe the average developer was aware of them at the time. I'm not saying you're wrong, but given even Scala didn't add support until 2010 they do not seem like something sorely obvious that should have been there from the beginning. In hindsight, sure.