You could also say that everyone moving to GitHub is leading to a dangerous centralized monoculture that evaluates software quality based on how well it fits GitHub's conventions.
git is still git - people don't even need a github account to have full functionality.
How many Github projects accept git format-patch submissions? There's functionality beyond the basic DVCS commands.
In Go's case, we will be using Gerrit (elsewhere) for accepting submissions. GitHub will only be used as a source code mirror, wiki and issue tracker.
Wait wait wait... no pull requests? There's no way you could just hook up PR's to gerrit? Using github just as a mirror seems to defeat the entire purpose of github, which is to encourage more community members to contribute.
Having migrated projects from GitHub to/from Bitbucket and internal git servers - which is a 10 minute job at most - I'm not terribly worried about this.