It seems GitHub is certainly king now... But how long will it last? I feel that Version Control is such a great market that surely some significant challengers will appear in the next few years. Thoughts?
They'll have to invent something really new or have an uphill battle, because GitHub now relies a lot on network externality. In simple words, it is the effect that you chose a popular network for the number of projects on it. I do submit bug reports to projects on BitBucket and GitHub, and not to private bugzillas/tracs (I'm tired of registering and confirming every time.)
I think if a new 'leader' in version control systems starts to make ground, github could easily adapt and support it. The github magic isn't just git, it's the network and collaboration tooling around your code.
GitHub makes very easy to collaborate to projects. I think that what makes them different than others such as BitBucket.
I can't find it right now, but some time ago I've read in a darcs vs git comparison that depending on orders of patches git can sometimes yield two different results. Perhaps in future correctness in huge procjects will be an issue.
How many source code bases are larger and have more authors than the linux kernel?
I know that Facebook had a number of issues trying to migrate to git. Many of these issues came from storing big blobs and mountains of code. There are code bases bigger than the linux kernel, they are often proprietary though.
Microsoft's come to mind.
I recall Facebook's git issue revolving around the fact they store all their code in one repo, too.