> natural market by default, meaning you must use brutal authoritarianism to disrupt it entirely
Ok? If that's how we define "brutal authoritarianism," I guess I'm a brutal authoritarian. There's a natural market for mob hitmen (scarce + in-demand)—are you opposed to a "brutal authoritarian" crackdown on those too?
> Disrupting natural markets with authoritarianism usually ends up worse than any downsides of the decentralized market regulation of that scarcity.
Citation very much needed. Suppliers of lead paint, asbestos, ozone-destroying aerosols, contaminated foodstuffs, etc. did not regulate themselves in a decentralized manner. In fact, take virtually any toxic contaminant or hazardous product and you'll usually find that the market colluded to cover up evidence of harm, rather than "self-regulating."
> generally trends toward making the highest value messages for both the audience and advertiser
Absolutely not. Consumers do not exert demand for certain types of ads in preference to others. There's no mechanism for ads to converge toward high audience value. It's advertiser value that is optimized for, often to the detriment of consumers (e.g. advertisements for profitable scams, which have negative value).
Even if you want to argue that advertisements inform consumers to some extent, that's probably outweighed by the extent that they misinform consumers. Consider infomercial products: Regular kitchen knives don't need an advertisement because demand is inelastic. If you're cooking, you need a knife; nobody has to promote the idea of knives. But the "slap-chop" is a product with elastic demand, and thus the marginal value of advertising is much greater for them. Hence, they can afford to buy up huge amounts of ad space to drum up demand for an essentially worthless product. The advertising ecosystem has perverse incentive to promote scams.
> But to ban it outright would require an extreme policing of all speech at all levels of society.
There are a lot of noxious and socially destructive things which are not practical to ban.