Don't be dense. If you pay $5 per conversion, and earn $10 per conversion, you make $5 profit as an ad buyer. What happens if you have 2 conversions? 3? 20? You make more profit.
Alternatively think about the flipside, how in the world could you possibly think having a lower conversion rate is good for the ad buyer? A 0% rate?
If you pay $5 per conversion, it doesn't matter if you have 100 impressions per conversion or 10 impressions per conversion. You are pretty conversion rate insensitive.
I mean, obviously it's different at the extremes like 0% vs 100%, but I doubt header-bidding produces that great an effect.
Alternatively think about the flipside, how in the world could you possibly think having a lower conversion rate is good for the ad buyer? A 0% rate?