the biggest problem with tether in my opinion is that people don't actually buy it with fiat (USD). instead what they do is that they either use bitcoin to buy it and then use that to trade altcoins. or they use bitcoin to buy altcoins and as they trade altcoins that market tanks (like the entire 2018) then end up being forced to convert their coins to tether.
this also explains why they kept printing more tether during 2018. and also why it is very bad for altcoin market. the day when tether finally is shut down by the governments the altcoin market will be devastated.
First of all people buy Tether with USD fiat. The way it works is someone wires money to Bitfinex, and they want to buy alts not on the exchange. They convert the USD to USDT and then they can withdraw those tethers into another exchange like Binanace. When this happens the market cap of tether increases.
And in 2018 during the middle of the year or so the tether market cap actually started to fall. It rose during the first half of the year but by Oct it ended up actually going down because people cashed their Tether's back to Fiat.