By [all consumers' discretionary income they'd be willing to spend] I think the parent meant [all consumers' discretionary income they'd be willing to spend on

By [all consumers' discretionary income they'd be willing to spend] I think the parent meant [all consumers' discretionary income they'd be willing to spend on web content] which is obviously a subset of all their discretionary income. Marketers are peddling tangible useful items, and people will pay a lot more for that than for the ability to read given words on the internet.