Though in this case, it's likely that Twitter has repeatedly run internal studies about what a paid-tier would look like, and each time concluded exactly what the GP is passing on: that it would be a net loss because the most-valuable-to-advertiser accounts would be the first ones to pay.

Even just psychologically, it would be a major blow to their sales pitch, going from "pay $X to put your name in front of the top movers and shakers in the world" to "pay $Y to put your name in front of all the Twitter users too cheap to cough up $8/mo to not have to see you." Not hard to see that $Y is going to be less than $X... probably a lot less.