Generics was a big thing because it impacted how a lot of code was written. It doesn't mean people don't want other things in the language that is hell bent on ignoring any practical advances in languages of the past 50 years.
What you propose is to basically stop improving the language because people complain.
Sum types and product types are fundamental to a type system, the same way that addition and multiplication are fundamental to arithmetic.
You wouldn't design a language with only multiplication and not addition. You wouldn't design boolean operations with only `&&` and not `||`. You wouldn't design bitwise operations with only `&` and not `^`. You wouldn't design set operations with only `∩` and not `∪`.
The alternative to "ignore" here is "ignorant", so it seems a nicer intention that they were aware of the fundamentals and chose not to use them.
I think a really important insight from ~50 years of PLs is that recent language features (say: lifetimes, or dependent types) do not always correlate with practical adoption, security guarantees, low cognitive overhead, teachability, readability, fast compilation, mechanical sympathy, and other such goals.
You can totally argue that Go should have been designed differently, but it's much harsher and untrue to say the designers ignored the ideas you have in mind.