The thing is, I don't think that most third party app developers would have really been against this. Give them enough time (say a year) and they'd probably get on board and do this, such as showing ads and sending back to Reddit whatever "signals" Reddit requires to use the API for free.
I think this is definitely part of the story but if this was the only problem, then I think negotations would have gone differently. There's probably more to it than just this, though this is undoubtedly a big part of it.
I heard elsewhere that the app store doesn't allow you to renegotiate an app contract that you have on an annual basis mid-contract, which is why Apollo suffers from such huge loses. But Relay, an Android app, is going to try to work with the new pricing. I wonder if it's only the Apple App Store that has this requirement then and if only iPhone apps are affected?