No airline is going to give up prime time slots at Gatwick, period. If BA and Virgin aren’t going to use them in the short term, they’re going to lease them or slot sit (not required under current waiver).
I know the world is shrouded in negativity right now, but it will return to normal. Virgin have made it abundantly clear they’re keeping the Gatwick slots, BA won’t let any go and Norwegian will keep their entire set as long as they intend to keep the LGW base in 2021 (or earlier if demand picks up).
Any talk of Stansted suddenly becoming the airport of choice is pie in the sky. Airlines will always favour Gatwick over Stansted, this will never change.