Especially since for the migration example you can trivially set the ambitious goal by measuring not the % of users migrated (since you need all of them) but me

Especially since for the migration example you can trivially set the ambitious goal by measuring not the % of users migrated (since you need all of them) but measuring the time or cost of doing so; e.g. set an OKR to finish it in X days or under Y person-hours or Z dollars of cost, and pick the values of X/Y/Z so that they're not "safe" but rather the values where you expect a 70% likelihood of meeting that goal.