It's a little funny to focus on GitHub's fork/pull-request model if you're trying to critique their centralization and lock-in. Every single successful open source project — on GitHub or not — has a canonical "source" branch and some sort of organization/leadership that decides what goes in. I'd argue that GitHub's behavior here isn't a power grab so much as a reflection of reality.
The "real" lock-in is with the discussion model — on both issues and pull requests — and the organizational structure you're able to set up. It's easy to move the source code to another service. Heck, you can even email patches to maintainers of a project on GitHub without creating an account. It's not easy to move historical discussions/reviews or "commit bits"/maintainership roles, and of course you can't participate in reviews of an emailed patch that a maintainer PRs for you unless you create an account.
The "real" lock-in is with the discussion model — on both issues and pull requests — and the organizational structure you're able to set up. It's easy to move the source code to another service. Heck, you can even email patches to maintainers of a project on GitHub without creating an account. It's not easy to move historical discussions/reviews or "commit bits"/maintainership roles, and of course you can't participate in reviews of an emailed patch that a maintainer PRs for you unless you create an account.