That makes me skeptical of all of these (minus the wind currents in cities, that might have taken a little longer).
Here's my sanity check when reading something like this on hn: What do you have me believe about the founder/investors? I understand that it's fun and common around here to be arrogant enough to presume that other people are absolute idiots, who are incredibly bad at their jobs, but I am not interested. If all you can bring are "duh" ideas, then that's a red flag.
Unless you can bring really insightful ideas, I am going to err on the side of the people who put years of their time into it and the people who put millions of dollars into it
Are they still going to be wrong? Of course. Am I likely to think the sidechair hn commentator is simply missing something in the bigger picture? Yes (and I can think of multiple concrete things in this case)
I don't know if a silent, fail-safe, and efficient method of flight is physically impossible or not, but I do know this is low on the list of applications it would be first seen in.
EDIT: I'm looking at the air taxi companies this thread started with, and no, they have not solved any of the relevant problems.
It is not only not unique, but in fact extremely common for startups to be grifts around impossible technical promises, live a few years off gullible investors who have way more money than sens and/or for whom losing a few million dollars on a long shot is just as bad as me wasting a few dollars on a gizmo off Temu I know probably won't work, and which then die out because their ideas obviously couldn't work.
They even sometimes find a niche by pivoting to some vaguely related tech. Say, while flying taxis obviously won't work, a startup trying to build them might find itself developing into a small company building helicopter propeller blades for some specific niche.