No actual disagreement with what you're saying there, but a few things about your example. A. You were dealing with at least one competent engineer and B. you're talking about mechanical engineering. We both agree that you need to hammer out the details with the client, but many clients looking for software to be developed are not engineers and frequently not even all that technical plus the ease of rolling out a mockup or prototype is generally a lot easier in a pure software environment. So sometimes hammering out the details can involve a mockup as a version of the specification for them to pore over. Especially for UI and workflow elements. Beyond that, sometimes dealing directly with the engineers whose requirement it is can be a bit of a luxury. Often you're dealing with one or more layers of middlemen. Worst cases can include your salespeople working out the requirement with their salespeople. If you lose the battle to have at least a constructive conversation over their heads, you're generally stuck with the sure and certain knowledge that there's some fundamental misunderstanding in the middle.
So, that's really the comparison I'm making here. In the traditional paradigm, you ideally have competent engineers/developers or even technically inclined end users who can work everything out. This new paradigm threatens to have the "developer" become the one who has no idea about what has been asked for, what it would really be used for, how to ask the right questions to improve the requirements documents, etc. They just become a middleman who feeds that into the AI.