The problem is that people who are willing to pay $20/mo for privacy are much more valuable to advertisers than those who can't.

If those users go away, the average $/user from ads goes down, because the only people seeing ads are those who are too poor to avoid it.

To an advertiser, poor people who direct their attention towards whatever is put in front of them are worth a lot less than rich people who carefully curate what their attention is directed towards. It's grody, but that's the advertising industry for you.