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.
Conglomerates spend oodles of money on advertising to maintain their brands as well. They have to. Ban on advertising would dramatically shift the balance of power. More innovating and better products can come to market without needing as much capital. You’d still have channels to review and inform about new products, which would now be a (more) fair competition.