Tried cloud9? It's actually very mature. The reason it's not widespread is because it doesn't offer any significant advantage for most developers. What's the advantage? Develop in the cloud, your environment everywhere? OK, so maybe it's an advantage for developers who don't know how to use ssh/scp/sshfs. So, for most talented web developers, the additional problems being solved aren't there.

It does solve: hey, I'd like to be lazy and develop with my stock chrome book. It does solve, hey, I'd like to develop without installing software (I think schools will be big customers, especially to get all the setups at once!?) but your typical good developer, is going to be very attached to vim, emacs, sublime text, or whatever he normally uses that isn't part of cloud9.

For developers of more complicated software, embedded, OS, etc. Or even for mobile devs, cloud9 is a non-starter since it doesn't integrate with their physical environments (Like, their ARM board, or iOS device, nuclear reactor, or whatever) so for a wide range of things it's just not an applicable solution.

I think cloud9 will see success in education markets, as well as business from developers who do lightweight web dev work who don't have a favorite editor already.

web based WYSIWYG document editors are mature, yet MS Office is just fine