People who use third-party apps are the most likely to be invested in the platform but they don't matter at all to reddit, the company. I would assume that they are also the most likely to use old.reddit, the most likely to use adblockers on desktop, etc. Reddit would rather replace them with people that will just use the default mobile/desktop experiences because they're the easiest to set up. That is the gain. NPCs are easier to develop apps for because they don't care if the apps are actually good.