Streak Break Solution, an Alternative to Payment
It's kind of strange that Duolingo only offers monetary payment options for when you miss a day on your streak. Someone with loads of money can easily rig the system...pump up their stats.
Why not also add an option for streak break challenges? For example, a user misses a day, then DuoLingo offers a challenge, e.g., "obtain 100XP in the next 24 hours to maintain your streak/status."
This way for those who do stumble every so often, it gives them a chance to rebound. These sorts of rebounds may even strengthen or encourage the learning process.
I'd be willing to pay a monthly service to have this sort of feature, even with ads included.