I suppose you could do a DMCA takedown? but that would really seem like using a morally compromised solution for good, as opposed to the more normal using morally neutral solutions for good.

At that price any copyright lawyer will be happy to deal with the paperwork. You were making 2000/month, so the infringement is taking money away. For a share of that it is easy to register the copyright and then send the proper letters. Probably this will settle out of court where apple pays your lawyer from fund the developer would have got and then removes the app. you get nothing directly from the lawyer except that next months all your sales come back to you since there isn't the competitor.

> in no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery

Just because another app works similar to another app does not mean it's infringing.

YouTube presumably does what they do because they'd otherwise be sued by very large companies with lots of lawyers and money. (Whether or not those suits had any merit, they still take resources and are an effective threat.)

Apple is not in the same position: the businesses who would sue them to try to force them to enforce their copyright are much much smaller than it (and are additionally very reliant on Apple).

Youtube does an incredibly poor kids b at it and screws over content creators legitimately and legally using something under fair use