If the celebrities leave, Twitter dies.
Whatever else the blue checks are, they're also a solution to a problem for Twitter, and those blue-checks and their activity are a huge part of why everyone else engages with the platform. If they make people pay, they better hope the adoption rate is incredibly high among existing blue checks (who cares about the unknowns who pony up for it, in addition) or they're gonna be in for a bad time.
The people who won't get it for free (who have blue checks currently) are entry-level journalists with a few hundred followers and cryptobros. Both of those classes of people should have to pay.
A lot of hustlebros and cryptobros do pay for checkmarks, they just don't pay Twitter.
> "Stephen, I'm trying to market your books, but the publishers aren't seeing any engagement on Twitter".
> "Oh I left because I don't want to pay $20 / month to someone I disagree with politically."
> "Have you been hitting the bottle again?"
End of conversation. The value most of these celebrities get vastly exceeds $20 / month. The Twitter-celebrity relationship is symbiotic, nor parasitic.