I am not arguing their decision is wrong on their real world system. It is a system design interview with a hypothetical situation completely unrelated to their

I am not arguing their decision is wrong on their real world system. It is a system design interview with a hypothetical situation completely unrelated to their prod system, where I suggest a different db than MySQL. They are the ones arguing that my decision is wrong and MySQL is the correct choice, at which point you have to defend your choices.