Just from using reddit, I can only really see a few ways for them to make money.

1. Subscriptions/awards. Not many people do this, certainly not enough to keep the doors open.

2. Advertisements

3. Selling user data

Let’s start with 2. The reason they re-designed the UI in both the app and the desktop version is because they need to create as much space as possible for them to put ads into- and still have it not be so annoying for the user that they stop using the site. Now, on the website they can still put adds on old.reddit, just not as many- so they haven’t come for that yet, because it isn’t draining nearly as much income as the mobile market. Their new mobile app does the same as the frontend redesign- it maximizes ad space, and also allows them to collect other user data such as location to sell to marketing agencies.

ALL of the alternative Reddit clients (or at least, all I have used) have adblocker built into them. For some of them, you pay the app for that- a payment which is often less than Reddit Gold is, and is usually a one-time payment. And these apps hold the user data that can actually be sold, like location. So third-party apps disrupt all three of Reddit’s possible revenue streams by having people not pay for premium to hide ads, by blocking advertisements anyway and denying Reddit the ad revenue for them, and by keeping the user’s data away from Reddit.

That’s why I think they made the API price so ridiculously high- it isn’t just meant to scare them away, it’s meant to be a reflection of what they feel they are losing in revenue from users using third party apps. If it was just about any one of the 3 points above, the rate would be much more reasonable- but it’s all 3.