Plus, good restaurants want to serve fresh tuna so most of it would have to be cut up and served that day. So the demand must be there, somehow.

Or, maybe this is an expensive marketing stunt. The guy who bought the big tuna is the owner of a chain of moderately priced conveyor-belt sushi restaurants; hard to imagine he’s pushing $60 nigiri through them. He does say that he might’ve gone a bit over (https://www.cnn.com/2019/01/05/asia/giant-tuna-sets-record-a... he was expecting to pay $300-500K USD.