Prisoner's dilemma: If only one ad-supported site switches to pay-per-view or a subscription model, people will go to the other ad-supported sites instead. It would take all of them to switch.

And if that happens, the experience would be strictly worse for the consumer. Every individual is going to have access to less content because they're not going to subscribe to every site that might conceivably have an insightful article at some point.

What does work is patronage-supported content, but only for certain types of content. For example, if Youtube were to remove ads tomorrow, I would expect most channels with original content to survive on crowdsourced patronage (i.e. Patreon or similar), but I would definitely not expect Patreon to be able to support investigative journalism. (It would be great if it were! But I don't see it happening.)