Seeing as business class seats take up
the space of at least 3 economy seats it would not make economic sense to have that real estate selling for less than 3x economy prices.
Aircraft that are already configured for business class will need to have that cabin filled, and selling at a discount is better than having it empty. As things move into the new normal, airlines will be able to assess what the demand is for business class and price it accordingly. It’s quite possible that the back few rows of J class get removed and replaced with economy or premium economy seats.
The prices of tickets will be all over the place at first while airlines try to gauge supply and demand after a complete reset. Booking and fare management systems will take a while to settle down.