[scholia]

The marginal cost of verification could be zero. All Twitter has to do is to get together with Facebook, Google et al and agree to support a number of approved independent identity systems such as Yoti [1]. That uses a combination of government documents (eg passports) and biometrics.

Basically, users would be able to sign up for a sort of "digital passport" ID that would, in the long term, be accepted by governments and most websites. Enough people might find it convenient enough to sign up.

Twitter already allows you to block all users who have not registered a phone number and not uploaded a photo or whatever. I used these settings. They actually do remove a lot of bots and bad players. Again, in the long term, Twitter would give people the option to see only verified accounts.

I really wouldn't care if it meant I only saw tweets from a relatively small number of verified accounts. It might be more like Twitter in its first half dozen years, when it was much more engaging and fun than it is today.

[1] https://www.yoti.com/

Yoti also works offline. For example, you can use it to prove that you are over 18 without giving away your name or other irrelevant information.