These topical experts as you put it, make more than $8/month today from their engaged audiences. What Twitter should do is build better engagement tools for them and then monetize that at much more than $8/month.
You think Stephen King, who is worth $500m, is going to drop Twitter for $96/year. That tweet itself was him doing a good job of using that platform (twitter) and his audience to get some free exposure.
Lol, no they don't. They are not rich and they live on fixed or unrelated salaries.
Stephen King is not subject matter expert. He is popular writer. He is also quite atypical in that he is so popular, then he really don't end twitter engagement all that much. I don't know whether he will ultimately pay, but he actually don't have to.
You think Stephen King, who is worth $500m, is going to drop Twitter for $96/year. That tweet itself was him doing a good job of using that platform (twitter) and his audience to get some free exposure.