$21 Billion USD is very large and USDT can indeed reach that value, especially since the price of Bitcoin is really in great demand by all investors, so all large fund owners from around the world can buy Bitcoin even though it is expensive and raises a large market cap for various stable coins.
I am also sure that Tether will continue to increase its marketcap as long as there is a lot of interest in crytpocrurrency around the world and I have known about this since 2016.