Why is it an initial assumption that customer acquisition and ad spend are linked short or even medium term? That doesn't seem right.
When I see an ad for washing machines it doesn't make me run out and buy a washing machine. It gets me familiar with Samsung the brand so in 6-12 months when I buy a washing machine I am familiar with Samsung and buy their brand. Or it means that if my washing machine starts to sputter I'm more confident knowing where to go to buy a new one.
It doesn't seem enough to stop advertising for 6 months and claim no damage is being done. It seems well understood that repeated exposure to a brand has a powerful effect on humans.