Even if Github did provide that setting, as a courtesy, someone could clone / fork the code to another repo (if you use any licence that allows it) and not enable that setting.
Sure that's possible, but there's a huuuge difference between Possible and Default Behavior.
In a case like this, GitHub itself could set up a bot account that forks all projects as soon as you make the switch. The company in fact would be incentivized to do so.