Many people asking about investing on tether USDT for long term holding because they actually don't know how tether USDT work. The fact of USDT it is not the coin for holding to get profit, but USDT are used as the stable coin that you can store your coins while waiting for another coin with high volatile in the market to drop in order to buy that coin through USDT at the USD rate. USDT price are tethering to USD price thats why USDT are called stable coin because it cannot be worth more around $1.
As you can see the USDT price chart above about its price movement are always go straight, so that's if you'll be holding USDT, you'll never ain nothing more than lose because fiat currencies mostly of the time they lose worth and as you know USDT are tethering fiat currencies prices. So that's USDT is not for investing, but it is only used for trading pairs safe coin which has no high volatile.