Exactly. Since we don't have hard data about how many people are denied boarding based on the tests (or data about cases onboard), the arguments are necessarily a little subjective.
That being said, testing seems like one mitigation where benefits clearly outweigh costs. The only people who are negatively affected in a tangible way are people who are trying to board a ship with an active case of COVID. That's the bottom line, but you rarely see anti-testing people trying to candidly defend getting on a ship with COVID.
You can do a proctored home test in 15 minutes for ~$20, which is not significant for anyone who can afford a
Disney cruise. There's some minor stress/anxiety for some people wondering how the test will go, but that seems like a minor intangible cost compared to reducing COVID on the ship.