When we're talking about the chances that an arriving fully vaccinated traveler could spread COVID-19, we need to compare the numbers to the numbers of false positives and false negatives from the tests that are being used.

There's also a resource cost associated with mass testing requirements. When testing capacity is limited, we could be denying tests to people who need them more, such as certain healthcare workers or others who have reason to believe that they're been exposed, or individuals for whom prompt diagnosis and treatment is especially important. Even if testing capacity is available and unrestrictive, all this unnecessary/unwise testing is still expensive and there are better ways to spend the money, even within the healthcare system, for example one could provide some free flu shots.