Yeah, I found Tether resilience incredible indeed, but Madoff and his Ponzi scheme went on for decades too even when JP Morgan figured out it was a scam and still did business with him while informing neither the SEC nor the US government.
https://www.justice.gov/usao-sdny/pr/manhattan-us-attorney-a...
So they must be "backed" by something, we just don't know whom or how, that allows them to keep their scheme running.