This is only true if you're on the free plan. If you get Duo Plus (family plan and split it, for example), all the IAP become obsolete. You can practice/learn a language 24/7 without limits. The gems become completely irrelevant. I think this is a really fair model and great bang for the buck.
The problem is when the whole thing is structured around gems, etc. Duolingo is incentivized to structure it in such a way that it benefits the nature of microtransactions. It diminishes the educational value of the whole thing, subscription plan or not.
