This has been quite the issue for a long time, and I can't think why the authorities are delaying their actions against Tether.
Madoff's Ponzi was pretty obviously a Ponzi and was reported for years as a Ponzi to the SEC and nothing happened until it was finally popped by the market retreating in 2008 and redemptions causing the Ponzi to go completely cash insolvent.
Tether/crypto is going to pop when it pops, and regulators will then step in aggressively afterwards.
Or it might not. Unlike what Madoff was doing, it's perfectly possible for Tether to keep their game up in perpetuity.
Madoff kept his far more ambitious and risky operation going for decades, I think Tether can certainly last for longer if not eviscerated by lawmakers.