Somebody would! (Or rather, some thing.) There are a lot of smartcontract liquidity pools in Ethereum that facilitate trade between two tokens. The set an exchange rate by a formula that only looks at how much of each token is currently in the pool, not at prevailing market price[1].

So if and when it's apparent that Tether will be worthless, there will be a mad dash to dump it in those LPs in exchange for the other tokens, which was what happened to TerraUSD [2].

Here's a list of the LPs that have Tether as one token, just on Uniswap:

https://info.uniswap.org/#/tokens/0xdac17f958d2ee523a2206206...

[1] This dynamic has the effect of writing a persistent ITM option with intrinsic value equal to LP's price divergence from the market exchange rate.

[2] Here's Curve's infamous 4pool, which has TerraUSD (UST) and three other non-defunct stablecoins -- it's 97% UST now: https://curve.fi/4pool

Both would happen simultaneously. Think of crypto like a balloon with only a narrow exit at one end going out into fiat: if you push down on Tether, BTC/ETH will inflate in Tether terms (because everybody who can moves into other cryptos) but deflate in USD terms (because there will be a run for the exits and a lack of buyers).

So both BTC/ETH and Tether would deflate in USD terms...