Gatekeeper has been around for 10 years and every year people have predicted you couldn't install software outside of the app store. It never came by. As a development machine, it just wouldn't work.
No. For a general purpose machine, especially for developers, you need the ability to access your files freely and to run any program. If that is no longer given, all developers and most users would drop the platform.
Also, it completely doesn't make sense as long as you allow virtualisation. Which Apple not only does, they even demoed this capabiltiy in the keynote.
