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.