Caustic is one major Android/iOS/Windows/macOS update away from falling by the wayside, and probably already has for some of us. It's really a shame that such an intuitive and well thought out app has been abandoned. It's easily the most streamlined music making app around. I've tried to move on since I know it won't be compatible with my devices forever, but nothing compares to this damn thing.
With that being said, I have a proposal for Rej. If you don't have the passion anymore, why not just let it go? Make it open source and allow the people to continue its development. It has so much potential to be a top mobile music production app. The workflow is there, not to mention the audio editor is one of the best around for mobile, and it runs pretty solid since it's all self-contained, just like Reason was before it could run VSTs. I'm sure there are people who use this app that are more than capable of keeping it going.
But this all just wishful thinking of course.