If whole European countries managed to sell their UST holdings somehow, prices would likely drop, treasury interest rates would rise, so mortgage rates would rise, but that's not what POTUS wants. Then the sellers would have mountains of cash that needs to go somewhere. European bonds are generally at lower rates than treasuries nowadays. So they'd have lower yields if the money stayed domestic. It looks to me like both sides loose. I guess this is why the treasury selloff hasn't happened yet, and probably won't for years into the future.