How much trading volume does tether have compared to "bitcoin + all the other cryptos"?
Answer: a lot, Tether is the unit of account. If something happens to it the crypto space explodes as the volume of real dollar trades in crypto is a fraction of Tether trades.
That would certainly be a novel notion of systemic risk. It's like arguing that the financial system would collapse if TSLA trading were halted tomorrow. There's no shortage of other liquidity channels between market participants, so the loss would be confined to the held value of the security itself and any of its derivatives. I'd argue that the systemic risk of my toy analogy is actually worse than Tether, as the derivatives tied to its debt and futures are almost certainly a huge multiple of its notional value. I would venture that isn't the case for Tether, but I suppose it's possible too.
Answer: a lot, Tether is the unit of account. If something happens to it the crypto space explodes as the volume of real dollar trades in crypto is a fraction of Tether trades.