That may be true but cruises are very different than other tourist destinations. It is much easier to spread diseases on ships even before COVID times.
Neither one of us know what will happen when it comes to travel but not everyone is eager to have the vaccines and some just can't or shouldn't have it right now. I hope they have a testing requirement option personally but if they don't they just won't get my dollars...
But so far nothing that I have read seems to indicate it will be vaccines only. I of course could be wrong but so far the discussion has always included testing as an option as well. Especially since there are a lot of Europeans who won't get the vaccine or who won't have access to a vaccine anytime soon.