And my point is that there's about a zero percent chance that the person making that decision, or even giving significant feedback about it, is going to be within 20 feet of the people in question who will be implementing it.

Or more practically that they would do a roll-out release (as they did) and increase capacity as they go. All questions for devops, the product manager or some type of an "Architect" working with analytics to estimate demand and volume.

To imply that a single engineer in a room with no information about Gmail's infrastructure is supposed to know that without (any information about: technologies, storage, indexing, user volume, mail volume, etc) seems absolutely fucking stupid. If they want to hear me think aloud about these things and know that I can be cognisant of them... well... there are better ways of doing that than asking me stupid questions that it's stupid for me to even try and answer.

As someone else said, simply changing the question from "How many gas stations are there in NY" to "How would you estimate the number of gas stations in NY" makes it entirely different, IMO.