The boundary between useful advertising and word of mouth is exceedingly thin and contextual. When Uber turned off $100m of advertising and found no meaningful change to its installation growth, had it already won? Clearly not then and not now; it's barely posted an operating profit and it still a couple billion away from positive net income. Rather, it's in a duopoly in the US and an oligopoly in some other markets and facing a pretty gnarly advertise (defect) or not (cooperate) prisoner's dilemma. Word-of-mouth can be a form of promotion. Most products have obvious entry pathways which smart and experienced marketers will readily identify; few of these are display ads, fewer still are billboards.

The space of advertising decisions is high-dimensional and dependent on too many factors to succumb to generalization. Many awesome businesses have thrived without it; others would've starved without it.