So if ad revenue isn't going to do it. Then it needs to be a freemium model with subscriptions. They probably get 25% of their revenue from reddit premium. That also means, charging for the API.
The problem for many of the third party apps is they've also been running a freemium model with very low overheads due to the fact Reddit has been largely funding their freemium model. They've been charging next to nothing for their premium options ($1.50 a month) and they have massive amount of very active users. They're so big that paying a reasonable per API request fee results in a massive bill. A bill they can't pay due to a freemium model and super low fees that don't pay for the freemium users. Third party apps directly compete with Reddit, therefore it's fair they pay Reddit for the resources they use as well as the lost potential of the users they did take. Those paying for premium on an app would probably pay premium on Reddit.
Then comes in the low value users, who are outraged the free toy they have wants to make a profit. "It's too much money" - funny enough Reddit users think they're worthless - was one of the main cries. "This third party app IS REDDIT" - well if they're Reddit they shouldn't need Reddit access. A userbase who resists being monetised. Either they're monetisable or they're not. If they're not the company can't survive. What the Reddit users need/want is a non-profit.