I must agree if Tether goes down, this will be a big problem for all cryptos because almost all stablecoins are in Tether.
A sudden Tether crash will lead to executions of orders with 20%-50% in all major exchanges, which will lead to mass disruption. Some people will buy coins 20%-50% cheaper, but the consequences will be harmful.
I believe after that the whole market will recover in no time as there are many stablecoins out there that willingly replace USDT position as the most used stablecoin.
It's not like the whole crypto market is entirely depend on the existence of USDT remember the day before stablecoin even a thing, and to find replacement for such thing is definitely very easy. Though that's just my guess without considering all other factors that might be unknown to me deeply.