I think the rationale is that while a few users might leave, the vast majority doesn’t care enough to make changes. With most 3rd party apps gone, more people will use the official app, providing Reddit with a lot more very valuable data they can sell while showing more ads. All this directly benefits Reddits bottom line. We live in a bubble where the exodus might appear much larger than it is. Most users just want their silly memes. The few tens of thousand users who left are barely noticeable. Major subreddits shutting down is actually annoying and that’s why Reddit is fighting against that.