> presumably get their revenues from that discretionary income

This is where you made a mistake. Their revenues comes from spending on necessities, not from discretionary income. Proctor & Gamble's $7 billion a year in advertising is coming from our buying soap and laundry detergent and toilet paper, not what you have left over for movie tickets. The $30 billion a year in medical marketing is coming from insulin and blood pressure pills, not your Netflix subscription. Most consumer spending is on necessities (food, housing, car payment, health/auto/home insurance, mortgage interest, fuel, etc) and those companies do the bulk of the advertising. People cannot choose to spend that money tipping websites instead unless they stop eating, stop taking medication, etc.

I don't think that distinction is precise enough for this. Sure soap is a necessity, but if you buy some premium fancy soap from P&G that had a lot of marketing budget contributing to its high cost rather than the supermarket brand plain soap, then plenty of that soap money was discretionary.

This extra precision doesn't change anything. None of the proposed alternative monetization schemes have suggested you buy a cheaper brand of soap so that you can put another nickel of discretionary income into tipping websites. Even some of the money you spend on the generic store brand ends up in someone's marketing budget.

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.