There's little I've seen in RubyMotion that provides a substantive improvement on ObjC, and a lot that is worse than ObjC.
For 90% of the fairly-standard apps that we'll ever develop, the overhead and headaches and needless code-bloat that Objective-C brings to the project is only a reminder of the elegance of Ruby and the beauty of simplicity. (It's also a reminder that you'll no longer need to budget for XCode crashes every hour.)
To write a 3D shooter you can use Objective-C specific features as much or as little as you want. For intensive performance code you can bypass the messaging system and write pure C, for the rest you can use Cocoa APIs. This is what you do in any language. Unless you want to argue that dynamic languages as a whole are not worth it.
If you are happy with RubyMotion go ahead, but don't preach bad mouthing a language you haven't even developed on.
Also, "a 20 years old dude comes with some hipster unproven technology"? Isn't this how Apple for one was founded? Not to mention Yahoo, Google and Facebook...
They are doomed to fail.