The folks on Twitter that rely on or improve their livelihood through Twitter (influencers, VCs, public figures, power users, etc), aren't going to shy away from $8/month. They're also the folks that produce the most content, and are most engaged. The blue checkmark and 'additional features' will appeal to folks who find these verifications and features useful. If you don't fall into this cohort, you use Twitter for free. It's not a bad business model, and self selects between casual consumers and content producers.