I don’t think you need a particular depth of knowledge. You need someone willing to do the pre-work -clearly there are notes from and conversations with the contributors beforehand, someone with curiosity to learn about things outside their areas of expertise. A good journalist would be fine
I agree. It's almost all prep. In fact that's exactly what I recall him saying in an interview once, that he still gets stressed by the amount of reading he has to do before each and every episode. He never winged it and rely on his knowledge alone.
And making broad connections across topics wasn't his style anyway. He's a legend but the show can totally go on without him, and it should.
I always hoped they would do a behind the scenes episode on what they do to create one episode - pre-reading, selection of experts, discussion of questions to ask etc.
He very often brings other knowledge to bear in conversations that clearly wasn't just prep work for the specific topic. He's not strong on science but he has a firm grounding in literature, philosophy and history that isn't common these days.