> I suspect the lack of sum types in many static languages are partially responsible for the popularity of dynamic ones.

I can totally see this. I started writing a small cli tool in Go, and despite knowing way less Rust, I switched to it and was able to make a lot better progress at first due to pattern matching and result types. It was just so much easier/more ergonomic to write a simple parser.

The Go code was a mishmash of ugly structs and tons of null checking and special casing.

I would say that traits are a better analogy for dynamic types, but at the same time you can think of enums as closed sets and traits as open sets, so they are different ways of encoding sets of possible structure and functionality, more alike in what they provide than it initially seems.

Some statically typed languages have union types that are extensible in a similar fashion; e.g. in OCaml:

https://v2.ocaml.org/manual/polyvariant.html

Very interesting perspective!