I guess there are 3 reasons why reducing ad spend might not show a negative effect: (1) Due to fraud, the money wasn't actually going to ads. (2) You didn't n

I guess there are 3 reasons why reducing ad spend might not show a negative effect: (1) Due to fraud, the money wasn't actually going to ads. (2) You didn't need the ads. You were over some saturation threshold and the law of diminishing returns kicked in hard. (3) The ads were valuable to you but not in an easily measured way. Maybe you could coast on brand awareness inertia for 6 months or a year before it affects sales. This case seems to have decent evidence for at least part of it being #1, but not necessarily all of it.