Take each one of the examples I gave and apply the premise:

"Islamic extremism presents an existential threat to Western Civilization."

For example, if a person has fought for Daesh, then that person would automatically be stripped of his/her citizenship, deported, and would never be allowed to enter a Western country again.

Give the same consideration to national security as you would to your home security.

Would you let a Salman Abedi, Khalid Masood, or a Khuram, each of whom was known to have radical Islamic live in your home?

If your answer is: "Yes, I would allow such a radical Islamist who practiced Sharia Law to live in my home," then
we have no basis for further discussion.