This is complex topic but I believe there are many ways trump can impose ban to tether like not allowing the use of dollar to peg their USDT and I agree that ETH value will plummet if stablecoin suddenly vanish.
Otherwise when will bitcoin gets its own stable coin on its own layer 2 to compete with ethereum? Layer 2's none of that sidechains nonsense that leverage no security from layer 1's.
There are plenty of L2 with its own stable for bitcoin, but never truly get the spotlight because people trust bitcoin much more than those L2 and if they want to do smart contract related activity they go to ETH where liquidity is not scarce compared to L2.
If trump bans Tether then they will losing funding from them.
Theres Layer 2's and their Sidechains. Layers 2's are less risky because they leverage the safety and security of their underneath main Layer 1 base chain network. Is there a stablecoin on the bitcoin layer 2 yet like lightning stable coin? Obviously they cant be a stablecoin on the main layer 1 bitcoin network because fees will be expensive plus its going to clutter the main bitcoin network. The bitcoin maxis want their main btc base layer to be clutter free so less storage space in Gb is required to run a node.