I would be kinda shocked if cruises DON'T resume in 2021, none of these cruise lines will survive if this shutdown goes until 2022. At least I don't think so.
I think by early 2021 there will be a vaccine and by summer 2021 it will be at least somewhat widespread. And as others have mentioned, a vaccine is not the end all be all, however it is very helpful. Covid will be a thing forever and all we can do is do our best to mitigate the risk. I anticipate a vaccine will help and who knows maybe it will even be required.