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?

Mockup is great if you are just working on the front end, but what is the save button actually supposed to do? I can figure that out, talk to the users and understand what they want to save, but I feel like that's the product owners job.

I can't tell if you're missing the forest for the trees here, or trying to embody the exact annoying response I'm talking about.

We'll need to escalate to the PO of Poe's Law to get a more precise spec.

I'd better watch or the two of you will tell someone else to write a funny comment and then use that as evidence for how hilarious you are.