Isn't (or maybe "wasn't") the intention of the check mark to differentiate legitimate public figures from those who would attempt to create fake profiles and impersonate them? Lately it seems like anyone who has had any sort of notoriety has been getting one, but if you could just pay for it instead that would completely defeat the purpose.
If you're a big enough public figure to want/need the blue tick, you're arguably making commercial use of twitter already, so you probably have a marketing budget that can sink a thousand bucks a year into maintaining your verified status with Twitter (in addition to proving your identity in usual ways).
If some famous person doesn't want to pay, that's fine: either quit using twitter's platform to market yourself, or continue but live without the tick.
And a few have become the victims of imposters who attempt to disrupt their careers by taking on their identity and messaging comedy clubs or TV networks or whatever. I don't think they are an untapped revenue stream.
Who's that?
"The blue verified badge on Twitter lets people know that an account of public interest is authentic.
We approve account types maintained by users in music, acting, fashion, government, politics, religion, journalism, media, sports, business, and other key interest areas. If you believe your account is of public interest and should be verified, this article outlines information about submitting a request."