The danger of a proprietary centralized service is directly proportional to how irreplaceable it is and how difficult it is to get relevant data back out. By this measure, GitHub poses very little threat.
Because it's DVCS, the code already exists on the author (and likely dozens of other users') machines. With the change of a remote GitHub as a source host can be effectively replaced.
When it comes to the non-source-hosting capabilities of GitHub, sure they're proprietary but they too can be replaced. GitHub has gained dominance in the open source community because it's very good at what it does. If it ceases to be the best place to host open source projects, people will stop hosting open source projects there.
That being said, the authors of awesome open source projects have zero obligation to put their stuff on GitHub if they don't like it. They just might miss out on contributions of members of the community who don't want to jump through extra hoops.
Because it's DVCS, the code already exists on the author (and likely dozens of other users') machines. With the change of a remote GitHub as a source host can be effectively replaced.
When it comes to the non-source-hosting capabilities of GitHub, sure they're proprietary but they too can be replaced. GitHub has gained dominance in the open source community because it's very good at what it does. If it ceases to be the best place to host open source projects, people will stop hosting open source projects there.
That being said, the authors of awesome open source projects have zero obligation to put their stuff on GitHub if they don't like it. They just might miss out on contributions of members of the community who don't want to jump through extra hoops.