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.
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.