Only linear broadcast was ever fully ad-supported
Quote
The only two things that were ever fully supported by advertising only were broadcast, linear broadcast radio and television. And those, like you said, the inventory constraint, the scarcity of the number of hours in a day.
Episode 267: Chocolove (episode page, opens in a new tab) 01:00:27 (play audio from this timestamp, opens in a new tab)
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Summary
Newspapers and magazines had subscriptions and classifieds on top of ads. The only media ever supported entirely by advertising were linear broadcast radio and TV, and that worked because there are only so many hours in a day, which made the inventory scarce. Podcasting doesn't have that constraint.