> "Do X or I'll quit" is a childish way to negotiate
I disagree with this. It's really the only way to negotiate, pretty common. The wording is important here. If you word it as "Do X or I'll quit" it sounds childish, if you word it as "I believe the company should X, otherwise I can't see myself working here comfortably and will have to consider my resignation" it sounds professional, but the idea is the exact same.