I appreciate Twitter has momentum, but no service is guaranteed to survive. It's largely a matter of who uses it and that can soon change. Twitter took years to make a profit, but a Steemit-based service may achieve that much quicker. They could take a cut of each post to cover their costs.
It's still early days for Steemit really. I don't know how big a team is working on zappl. There are challenges in building a site and/or app. Meanwhile Steemit can still be growing. I just wonder if/when that growth will really take off.
I'm no expert in this stuff. Just find it interesting.