How do you separate "people who haven't seen an ad" into a group in Google analytics? I don't believe "organic" source does that, because that means they weren't targeted by the ad in the first place, creating a bias.
The flip side is that not every campaign, targeting strategy or ad will be incremental. You have to test and retest your way to it. A blanket assumption won’t work.
The fact that Web and AdTech allow the appearance of individual-level targeting does not necessarily make that a Good Thing.