[Posting via gmane, followup seems to be broken.]
From a purely pragmatic perspective though, "Import from GitHub" is already one of the features offered by GitLab, and will no doubt be a standard feature of any other open source repository management systems that gains widespread popularity in the future. Donald also noted a while back that this is a reasonable lesson to take away from the Linux kernel Bitkeeper experiment - they used the proprietary service for as long as the vendor was playing nice, and when the relationship soured, they migrated away again.
I think this is different from BitKeeper: GitHub has a dossier on every participating OSS developer, including contribution statistics, political correctness on the tracker, age estimate from pictures and so forth.
Before GitHub, all this information *could* have been gathered on any individual, but at a far greater cost in time and effort.
If GitHub turns evil, it will be most likely by selling this nicely packaged information to recruiters. It's too late to move away then, the statistics will have been collected already.
Quite honestly, I'd feel like working for Andreessen & Horowitz for free if we move there.