While you are right, there is more than just the code. In general one should be wary for lock-in when sculpting their development processes around the features that GH offers, i.e. the Issue tracker, Projects & Teams, Discussions and not to forget GH Actions. And besides that there is FOMO for many projects to leave GH due to its popularity. Does your contributor base move with you to another platform? Is it as easy to find new contributors, or even to find the project itself?