Yes, there will likely still be ways for a malicious USB device to use electrical side-channels to attack a connected computer. But devices like that will be much harder to develop. And more importantly: compromised devices which weren't originally designed to do that, won't be able to rewire themselves into side-channel-exploiters. So if my USB storage device has a firmware vulnerability, and a malicious computer reprograms it, it won't be able to use electrical side-channels to attack my other computers because it doesn't have a suitable DAC and ADC.