I've never understood the idea of a 'teaching' programming language. (Modulo examples like Logo and Scratch, which offer much more than a language as part of a wider teaching/computing system.)
You spend all this time ramping up on a language, toolchain, library, etc, that you will eventually be unable to leverage beyond a certain point since it is not one used by everyday programmers. The delta in "quality learning" between a teaching language and a non-teaching language would have to be astronomically high, imho, to offset the difference in capability and the opportunity cost of learning an applicable skill to a wider problem set. Something that is just a slightly better Python or a slightly better Clojure for the purpose of illustrating computer science concepts seems like something that you'd be doing a disservice to students to teach, vs the "real thing" which they could carry for the rest of their lives into future projects ad infinitum.
You spend all this time ramping up on a language, toolchain, library, etc, that you will eventually be unable to leverage beyond a certain point since it is not one used by everyday programmers. The delta in "quality learning" between a teaching language and a non-teaching language would have to be astronomically high, imho, to offset the difference in capability and the opportunity cost of learning an applicable skill to a wider problem set. Something that is just a slightly better Python or a slightly better Clojure for the purpose of illustrating computer science concepts seems like something that you'd be doing a disservice to students to teach, vs the "real thing" which they could carry for the rest of their lives into future projects ad infinitum.