Why do you assume that any alternative to ads has to be centralized? You also seem to assume the advertising space is not centralized (lol).

If we can build a (centralized or decentralized) system capable of querying/serving content based on price and ad revenue projections, we can also build system capable of querying/serving content based on relevance, ratings, reviews and any number of parameters.

You seem to imply that advertising and marketing give us some kind of advantage in terms of (de) centralization, but that's really not true. The whole purpose of advertising is MAKE people look at ads despite the fact that almost nobody actually enjoys or values looking at ads, whether it's run as a centralized or decentralized model is an implementation detail.