Fixed timeline. Fixed resources. Fixed scope. Pick two.
Timeline. Resources. Scope. Everyone wants to control all of them. Have 100% certainty. Especially management. They want to know the date you’re delivering the exact feature set and you can’t ask for more people.
Bad news. In product (and probably everywhere else), you can only pick two.
If you want a committed date (fixed timeline) and you have limited people (fixed resources), you have to accept a variable scope (aka, we're going to cut stuff out).
If you want a fixed timeline and a commited feature set (fixed scope), we're going to tell you the resources we need to deliver it (variable resources).
If you want a fixed scope and limited resources, we're going to have a variable timeline, and deliver whenever we can.
If you try to fix all three, you not only lose the opportunity to build a product that works, but you also lose the team.