Mike the Millennial is strapped for cash, and so won't pay for content directly. Alice the Advertiser wants to convince Mike to buy her products, so she places ads with Nick the Newspaper. Nick gets money from Alice, to provide Mike with content for no cost.
But doesn't Alice expect a return on that advertising spend? Doesn't Mike ultimately pay Alice more than he would have if he had just paid Nick directly?
I know it's a hell of a lot more complicated than this, but aggregated over all advertising industry revenue, and aggregated over all content subscription revenue, it seems that the ultimate price of advertising (once it percolates down to the consumer) will always be greater than an equivalent cost of subscriptions.