First question that begs to be asked, why would you want to mid/long-term hold USDT instead of BTC or FIAT?
rationally, you wouldn't. there is simply too much counterparty risk in bitfinex and tether. you shouldn't hold USDT for the mid/long-term for the same reason you shouldn't store funds on exchanges for the mid/long-term. tether could be shut down by regulators or fined into insolvency, they could exit scam or get hacked. it's much more sensible to sell on the exchange (or OTC) and wire USD into your bank account.
the exception is those that are dodging taxes or KYC. if you need to hedge the USD value of your coins without leaving paper trails to your bank account, USDT markets can be an option. binance comes to mind.
Second question, who would want to hold 2.5B USDT? Where is 2.5B USDT stored right now?
i imagine exchanges hold most of it. binance and a slew of other altcoin exchanges have fairly liquid USDT markets.