If people believe in USDT then it's the same as USD almost. There's far more than 2.5bn of USD being held across the globe so 2.5bn isn't really surprising. Many may leave USDT in their accounts in order to capitalize on short term volatility that they would not be able to do so if trying to use fiat because of the delays involved. Given that the whole market cap is 200bn it means only a little over 1% is in Tether at any one time, personally i don't feel that is too much at all.