I think we are talking past each other a bit, as I completely agree with

> it is not necessarily true that something with a complex implementation has a complex interface.

In turn, my point is that as a language implementer you only have so much time available to work on the language and surrounding ecosystem. If your time is spent dealing with a complex spec, compiler and VM then you have less time to spend on the rest of the ecosystem. I think go has shown that spending tim on the surrounding ecosystem can be very valuable (e.g. go fmt, go vet and more recently go fuzz).