I think you are vastly underestimating the amount of people that only interact with social media through apps in this day and age. Even on Reddit I would be surprised if the old design is more than a few percent of the user base.
It honestly does raise the question of why they are still maintaining the old UI. It seems like it would be an annoying legacy product to keep alive for a tiny part of the user-base. Perhaps it’s used as an API stability canary or something though.