If you look at the iron triangle scope-budget-time you could argue that budget and time are fixed…

If you look at the iron triangle scope-budget-time you could argue that budget and time are fixed with Scrum (team size generally is stable and you can determine a number of sprints). Scope is variable though. This is what Scrum is about. Empiricism. You might have an idea about the size of a required feature, but you learn along the way.