The idea feels like sci-fi because you’re so used to it, imagining ads gone feels like asking to outlaw gravity. But humanity had been free of current forms of advertising for 99.9% of its existence. Word-of-mouth and community networks worked just fine. First-party websites and online communities would now improve on that.
The traditional argument pro-advertising—that it provides consumers with necessary information—hasn’t been valid for decades.
The hypothetical was a world without advertising, not a world without capitalism. You could make a food blog for the love of it without caring if anyone reads it or not now, but you likely wouldn’t bother with SEO if you did for the same reason.