I think the precruise tests will continue not in spite of cases being milder, but because they are. The few cases that involve more serious symptoms, hospitalization, or death will stand out and become newsworthy. Precruise testing, and other screening, will discourage many people who suspect they are ill from trying to sneak aboard.
The cruise lines will not want to have the rare extreme cases prove the truth of "one death is a tragedy, a million is a statistic."