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.

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.

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.