20-Year Prospect
It's impossible to predict the app store of a decade from now, let alone tomorrow. Think about it, the only game on a mobile phone twenty years ago was trying to play "Twinkle Twinkle Little Star" on the keypad.
Being a successful mobile developer today means staying on top of trends and practices on a daily basis. If you're already doing that, there's no reason to believe you won't still be making fart button apps for the Holographic Callzoid 3.X whenever it is that current smartphone platforms become oh so passé.
While we have no idea what the market will be like in twenty years, we can pretty safely say it will definitely still be around...in some form or another.