How would you keep track of the sales without cookies? Most people don't buy on the first visit.

Unless I'm misunderstanding, the technology described in the article is designed to fight against cross-site tracking (i.e. following users all over the internet to different sites)

I suspect few users would object to a single web property remembering them across multiple visits.

So you'll have to rely on the webshop to keep track of purchases? I'd rather have a third party involved that has to keep both the publisher and the advertiser happy.

That gets defrauded via a technique called "cookie stuffing".

https://en.wikipedia.org/wiki/Cookie_stuffing

...which can be stymied by using first-party cookies for lead attribution, which is exactly what webkit is recommending that sites switch to. Voila!