If they shut out third party apps from official APIs, that isn't game over. Those apps can still scrape HTML and while the new reddit frontend is modern and can easily combat this, old.reddit is an unmaintained static target.

They presumably know this or if they don't, they soon will. To avoid allowing app devs to run around the API changes they have to either continue maintaining the front end or cut the dead weight. Given their decision making lately, I'm going for the latter.