I agree, and then some. You can get consumers to buy more, but, ultimately, there are only so many possible sales, and further advertising won't push past that.
However, sellers are competing for those sales. This competition happens through price and quality, but also, and to a much larger degree than we'd like to admit, through advertising. After some point, you are no longer advertising to convince someone to buy a gizmo; you are advertising to convince someone who has already decided to buy a gizmo to buy your gizmo instead of their gizmo.
And so, everyone needs to advertise more, and more, and more, just to stay in the same place. The real winner is, of course, the advertisement industry, and especially Google who not only provides the ads but also owns the algorithm that implements that competition (what conflict of interest?).
A corollary of this is that the ad industry's typical claim that hard anti-advertising laws would depress commerce is, in fact, false. I would argue that ultra-hard enforcement of GDPR, for example, would actually benefit manufacturers of actual goods, because they would still reach the same sets of eyeballs, but they wouldn't have to outspend every competitor in a rigged auction every millisecond in order to do that.