What do you mean by USDT as 'money printing machine?' For all I know, the main catalyst of the 2017 bull run was credit cards being used to purchase bitcoin and the futures market, and USDT being there just to, well, be a stablecoin and nothing more. Right now, Tether is in hot waters for not actually stating or showing a proof that they indeed have that much money to match any issued USDT in the open, yet they are still continuing to do what they always do. Sooner or later, Tether will be involved in some form of 'hack' that could potentially hurt millions of dollars of money converted into them in the meantime.
Future market was a major factor in the bull as well as tether was instrumental in pumping the price so high, during 2017 there were a lot of tether being created without any valid proof on that they are fully backed by dollars, if there is any market downfall then tether will be the main reason for that and i feel that something big is coming that could wipe the market by a big margin. Credit card usage were there, but that was not the main reason for the big rally, it was minuscule when compared to the amount of tether being created during that time.