It's a for-profit company. The monthly subscriptions are so that the company makes money. The subscription gives you some extra features and removes ads. Seems pretty reasonable.

As for the gems, that's part of the gamification. You get gems just from doing lessons. You can spend gems on cosmetic things or on buying a "streak freeze" that lets you avoid losing your "learn every day" streak if you forget or otherwise can't use the app one day. Maybe you can buy gems too, I don't know, but they don't seem that awful. They're just nudges to try to keep using the app every day, and if your goal is to learn a language that's a good thing, right?

IMO gamification is good. Learning a language can be boring, especially when it comes to grammar lessons. Making it more entertaining means you're more likely to want to do it, so you're more likely to achieve your goal of learning another language.

Having said that, there is definitely enshittification going on. It used to be that the most of the program was available to people without a subscription, and only a few things were "paywalled". Now only the main path of the main course is not paywalled. It used to be that if you got bored with the lessons you were doing, you had alternative things you could do. Even the main lesson plan used to have optional paths. Now, unless you're subscribed the only two options are "stories" or the next lesson in the chain.