I don't see why a complex language can't be good for teaching if its designed in such a way that the complexity is optional, and you can use a focussed subset for pedagogical purposes (especially when the language supports using different focussed subsets depending on the specific focus of pedagogy.)

I mean, even if you look at how language like Scheme/Racket, or Python, or pretty much any other language is used in teaching, you don't usually use the whole thing in any one teaching context.