I don't plan on going back since I just can't condone how Reddit management handled the whole issue, but there is one thing I wonder why it is not a possible solution for 3rd party apps:
Wouldn't it be possible to ask the userbase to just get the API key themselves?
If every user of a 3rd party app has their own API key, they won't have to pay anything won't they, since it will be hard to reach the free tier limit.
And even if a user does reach the limit he can get a couple thousands API calls for just a small number of cents.
Reddit will be still getting the same number of API calls, but it won't be the responsibility of the 3rdparty dev but on each user if the limit is reached