I feel Reddit could have easily come up with a model where the API usage gets tied to premium account and that decouples apps from API charges.
Reddit has now created a new problem for itself which is that a huge user base that previously didn't bother to look at third-party apps has suddenly become aware of third-party apps. So now, they will start bypassing Reddit's official apps that generate revenue for Reddit in favor of third-party apps that seemingly have a better UX and are ad-free.