>Do you mean they could sell or design their own ads?

No, I mean they could host their own ads. The way ad-blockers work is that they see requests from a web page to other domains, particularly known ad-serving domains, so they reject those requests, take those elements out of the page, etc. But they don't alter self-hosted images. So if a web page has a bunch of JPGs which are really ads and not just helpful illustrations, and embeds those into the page, the ad-blocker has no way of knowing these are actually ads, and can't block them.

But these places never do this because they want to outsource the advertising to someone else, and the advertisers want to not just show ads, but also track how many times the ad was served, who it was served to, how long it was viewed, and lots of other information that isn't necessary for advertising (and was never a factor back when ads were simply printed on newspaper or shown on TV). In short, the advertisers insist on spying on people now. This should not be tolerated or excused, ever.