Apollo has no leverage here unless there is strong evidence most of the Apollo users will leave Reddit if the app shuts down. I don't believe they will. The other potential leverage is the upcoming subreddit blackouts, or hinting at taking the Apollo users to start a competitor. The developer said they are not going to build a competitor (that was a mistake, they shouldn't have revealed that card), so I think the blackouts are the only chance of lowering API costs.
If Apollo's userbase was actually generating $20mm/yr, acquiring Apollo for $10mm is a no-brainer. But if that were the case, keeping Apollo running as it is would also work.
Obviously this is not the case. Apollo is confusing costing $20mm with generating $20mm.