No, the point was to demonstrate how you’d design a complex system.

If a valid answer was “just use Postgres” then it just wasn’t a very good interview question.

In real life, the answer almost certainly would be “just use Postgres” and everyone would be happy.

No, it was a perfectly fine question IMHO. it is a broken incentive - it is expected that you design complex systems regardless whether they are useful or not. Try to interview for the role you have to fill, nor for a role you a dreaming you would love to have whenever you're Google2.

If the interview wants you to think about stuff that never happens in your role, I think it is a sign that in your role, you're expected to solve the problems like in the interview.