> I spent 18 months on it and the learning was soundly thrashed by three weeks in a classroom, and for all the pupils, not just me, primed for success by 18 months with Duolingo.

Yeah, but if you have time for 3 weeks in class, why do Duolingo in the first place? Those serve different markets. Duolingo is for low effort, low commitment, but interested enough to do it a bit in their spare time kind of people.