This isn't really true. The wholesale networks are operating at speeds that make this kind of sniffing impractical. I'm not saying they don't ever siphon traffic for LEO or other reasons, but not for privacy-violating/ad-targeting reasons. DO probably is running 40gbps alone with each transit provider, plus sending traffic over peering circuits, so it's just way less practical.

So what you are suggesting isn't actually true, and it's hardly hypothetically even possible.