> "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.

I would respectfully recommend that you read "Getting to Yes" by Roger Fisher and William Ury. Effective negotiation is often about expressing interests, not specific actions or ultimative positions.

> It's really the only way to negotiate, pretty common.

As I point out in another comment[1], not only is this not true, negotiation experts universally criticize this as a way to negotiate.

[1] https://news.ycombinator.com/item?id=25309003

> the idea is the exact same

Yes, in that they are both childish. Your version just added more weasel words. The second you bring up the "R" word as a threat you have played your hand; everything else is window dressing.

Better to word it as "I believe the company should X instead of the current W, and to that end I will work with Y and Z to come up with a plan for moving to W' and what that should look like, once that is complete I will have better clarity over how to get to (W+X)/2, or if perhaps some V would actually be better for all parties.

Is "do X or we'll fire you" childish? Because that's pretty much how most employment works…