Also mind: Even if pull requests are off somebody else might emerge and maintain a fork, which becomes more successful. Maybe even going as far that that the original owner can safely outsource the maintenance of the thing he created.
The minute they become just another dumb git host, their relevance disappears.
How do you get locked in? PRs, once merged, are just regular commits. You lose the history and discussion around the PR itself, but that's all, but it's not like any alternative allows you to migrate ( e.g. the abomination that is using email for git collaboration is stuck in unreadable, unsearchable, unindexed emails somewhere; you can copy the data pretty much the same way as from GitHub, but both are unexploitable without a lot of tinkering / adaptation)