You have too many misunderstandings about these concepts. First, if Trump bans $USDT, it won't directly affect $ETH. We might lose a lot of MC if that happens, but that doesn't mean it will directly affect $ETH (currently $USDT is used in many L-1 & L-2 projects).
Referring to the question in the title, I'll answer it with "Maybe", the $ETH ecosystem is indeed supported by stablecoins, but actually it's not only that, there are many other types of supporting assets that keep $ETH alive until now, so, even when stablecoins don't exist, it won't destroy $ETH.
Yeah USDT is mostly used on Tron so wont affect Ethereum if USA Govt. goes after Tether but USA Govt. can go after Asian Tron instead and that that will make all USDT holders swap their USDT for USDC out of fear I guess and finish off the Tron network unless Tron and Tether bring their own stablecoin for the chinese currency or whatever. Is that why China does not like Tether because it lends money to the USA Govt. and not to the Chinese Govt.
Okay apart from stable coins and NFT's, what other assets rely and need the Ethereum blockchain to survive?
Can there ever be a independent asset with its own independent decentralized blockchain like bitcoin? Like a dollar stable coin on its own blockchain without requiring a need for Ethereum or Tron to host it or is this simply will turn out to be a CBDC?
And talking of CBDC's can central banks take over the Ethereum and Tron blockchains if they decided for example to make USDC their official CBDC for the USA Govt. when USDC is still hosted on Ethereum?