What's the desktop GUI situation for Go these days? Is there a as portable as Go option?

Sciter: https://github.com/sciter-sdk/go-sciter

The best option now is gio https://gioui.org/

fyne: https://fyne.io/

nope

I have never seen a programming language prioritize & obsess over binary size like Go.

Is it really a big deal? Or is it more that it's a number so that allows people to scrutinize it more?

Go linker generates relatively big binaries due to static linkage. For some cases binary size is becoming an issue. 100+ MB binaries aren't uncommon in Go universe.

Can't you just use dynamic linkage?

I think it’s scrutinized more because the sizes can be on the larger side due to including the runtime and garbage collector

If you want to run them in an environment which reduces payload size (i.e. AWS Lambda) you really do, I'm afraid. This is possibly great news for Go users on AWS :)