You may have already checked out this post making the rounds recently from Robleh, one of the developers of Next Keyboard. Even I was surprised their revenue plummeted that quickly! I also thought the below quote was important to examine.
In hindsight, I didn’t realize how risky building a keyboard would be. I certainly did not know how complicated it was. Naivete is good sometimes, because you don’t get stuck in the trap of overthinking, but this time it bit us in the butt.
Third party keyboards still cause me trouble on my iPhone. Even the best, such as Google's GBoard, are limited by the framework and buggy. Adopting a new framework, such as iMessage stickers, can be great, but you have to think about the project as a whole.
How our app went from $20,000/day to $2/day in revenue by Robleh Jama