Twitter could charge for something. They already do (ads). However, some advertisers will always pay more to reach those on a platform like Twitter, especially

Twitter could charge for something. They already do (ads). However, some advertisers will always pay more to reach those on a platform like Twitter, especially those who are likely wealthy, and those budgets are large enough to overwhelm whatever you would pay to avoid ads. If, however, you are simply arguing that Twitter should add some cool features that you have to pay for, then you are correct. They could, and may.