They won't cancel it. WW don't sell return tickets, if you buy a return you are just buying 2 one ways in a single transaction. You are therefore not bound by any round trip fare rules but rather the fare fuels of the 2 single tickets you have purchased. What happens with "legacy" airlines when they cancel return portions is that you are usually in breach of the fare rules by not taking a sector in a return ticket. No such restrictions here and nothing to worry about.