Not sure I fully buy the argument that the industry is actually genuinely losing that $6bn. If I buy plates in bulk but find that 25% are usually broken in shipping, I will tend to pay 25% less for each shipment on the assumption that this will happen - the supplier might even have already priced this in.
Online advertising is a much more fluid market and very metrics-focused - I find it hard to believe that most smart companies haven't priced this into their bidding. There's not some sort of intrinsic value per click or impression - it should purely be based on the expected propensity of it leading to a conversion. Fake clicks and impressions are just a factor which reduces this and therefore it should automatically be taken into account.
In fact, I'm surprised it's not the suppliers of inventory, like YouTube, who are complaining that this is erroding their value. If advertisers are getting caught out by this then they're not being very smart.