You can’t just kill the traitors just like you can’t just imprison all the murderers and political dissidents.
I agree that it's impractical. If I could Thanos-snap them out of existence, I'd do it in a heartbeat. Unfortunately, such easy solutions are not possible. So if we ever do have any hope of recovering the ideal of a country based on the rule of law, we'll have to figure out what punitive or rehabilitative measure to take against a fifth of our population. Rehabilitation would be extremely difficult, though, as anybody with a Trumper for a family member will attest.