Why navies sometimes buy mediocre aircraft

What could the US Navy have obtained instead by cancelling the Buccaneer and Vindicator?

Very little immediately; they were the available aircraft and filled training and operational gaps until better types entered production.

Cancelling a weak aircraft does not create a superior replacement if no alternative production line exists. The Buccaneer and Vindicator could still train crews, tow targets, support Marine units, and let air groups develop. Their value was often as a bridge until the aircraft a service actually wanted reached production. The deeper failure was insufficient peacetime ordering, which forced wartime buyers to accept what factories could already deliver.