I have zero data to back this up, but I would say that this will damage you more than help.

If even a smallish percentage of key athletes/celebs/whatevers move exclusively to Instagram or something else, it would seriously disrupt Twitter. IG would love that, in fact.

Plus, how many people with over 10k actual followers are there really? Is $120/year times that number really going to make a difference for the business? Seems cheap and risky simultaneously.

It will be interesting to pull out some numbers here to see how the model looks. Is there a way to find out how many people have more than 10k followers? Politicians and celebs are getting access to their fans and followers. I think that kind of access should not be free especially when celebs are promoting their apps, music, etc.

Well, that might deal with "buying followers". But my personal pet idea for paying for twitter would be something like reddit gold: a slight premium tier, but not necessarily paid for by the account holder. Encourage people to pay for the accounts of people they like. Heck, provide a "micro" donation system alongside it (although this runs into problems, Twitch passim)

Also, a lot of people would like a better anti-abuse system and some might be prepared to pay for it.