There's a misconception about tanking.
It's not a coach going to his players and telling them to do less than their best during games. That would be leaked fast and would not go over well (ask the Miami Dolphins, it comes out eventually).
Teams tank by tearing down the roster so you can't win games. And it can work. The Philadelphia 76ers did it, though they didn't get the ultimate reward for it. The Chicago Cubs and Houston Astros did each win a World Series after a total teardown.