I think we are talking about different complexities of projects. In a complex project in a complex environment in a team larger than 2, a mockup often does not

I think we are talking about different complexities of projects. In a complex project in a complex environment in a team larger than 2, a mockup often does not cut it. I've seen underspecified requirements leading to misunderstandings countless times. I have never seen a situation where I thought that writing down those 5-15 acceptance criteria upfront was a waste of time. The discussions that follow when a deadline was missed because of misunderstandings feel much more like a waste of time to me. In a simple project a mockup can be enough, but then what do you need a PO for?