> But git itself is decentralized.
To me that sort of makes things worse. I have nothing against using GitHub as such, but I also think it's important that projects like this should lead by example. If you centralize things that are inherently decentralized it's hard to make a case that decentralization is important.
> many people (including myself) prefer the command line where possible, which is essentially universal and not hard to pick up at all.
As someone who, at least thinks they, understands how DVCS works I find the git command line quite unintuitive. It's a mix between more low level commands and added shortcuts. The solution becomes "just run these three commands you know and use Github".
> In a worst case scenario the entire project could be preserved with history and exported.
That's not really realistic, unless something really drastic happens. People often use Github for more than git itself, it becomes an integral part of the workflow. If it's was easy to switch people would run something else in parallel (which some projects do).
All in all I think the pragmatic solution is to have your own system and then interface Github as one channel. If you can't set up your own system that is competitive as a completment to Github, well then you are effectively dependent on Github.