It is hardly trivial to move off. Bugs would have to be migrated, links updated, documentation, automated build systems, workflow as well as user remapping.
While not being Mercurial is one thing it is really more about being on GitHub that is the problem. Kings will get complacent (remember how long it took to GitHub to add some features).
I also vaguely remember the github guys trashing Mercurial with arguabley a propaganda campaign (something "git is better than X"). All that content is missing now but I do remember (I'll have to look on way back machine later).
While not being Mercurial is one thing it is really more about being on GitHub that is the problem. Kings will get complacent (remember how long it took to GitHub to add some features).
I also vaguely remember the github guys trashing Mercurial with arguabley a propaganda campaign (something "git is better than X"). All that content is missing now but I do remember (I'll have to look on way back machine later).