Free Software is necessary but not sufficient
Recently Unity announced changes to their pricing model (archive) which have been very poorly received by their users, to put it gently. They have since backtracked (archive2) somewhat, but at this point it would not matter even if they completely scrapped their plans and went back to how things used to be. The trust has been broken and many game developers are waking up to the fact that Unity is effectively holding their project hostage. Switching from one engine to another is akin to a full rewrite, and depending on the size and progress of the project porting might not be a feasible thing to do.