Earlier quoted context omitted.
> They made him do it again and he passed. I would hire the "just use postgres" dude in a heartbeat without re-testing, if the numbers made sense, and perhaps give a stern talking-to to the interviewers. But then again I'm not a unicorn founder, so what do I know.
Eh, it's a good answer and shows good instincts, but they still want to know how he would design a system if one was necessary. There's no need to be ridiculous about any of this from either perspective, which is why it should never have been a "fail" without the original interviewer simply saying "That's a solid answer now tell me what you would do if you had to build something new". I mean look how much time he was…
This is the same issue that was prevalent when the industry switched from HDD to SSD: some system design questions suddenly became trivial, because the IOPS went up by magnitudes. This is not a failure of the interviewees, who correctly went with the times, but a failure of the interviewers.