There is no connection between Ethereum and the stablecoins on the basis of them keeping ETH alive. The Ethereum blockchain is just one of the blockchains in which those stablecoins run and without those stablecoins, ETH will exist. Likewise without the Ethereum blockchain, stablecoins will exist. Now we have more chains such as TON, TRX and others where more stablecoins are traded than ETH, yet ETH is still doing so well.