Basically: In some countries, the pledge is with the constitution or the people, but in others (like constitutional monarchies), its a pledge to the (constitutional) monarch and their successors.
What is your opinion on this loyalty pledge? Do you believe it’s a reasonable request?
(For context: My mother and older brother had to do the pledge to gain [US] citizenship so the idea of deportation isn’t looming over our heads. I didn’t have do it because I was under 18 and my mother’s citizenship status automatically carried over to me according to the law.)
Espionage also carries heavy sentences, and everyone knows you can be prosecuted for plotting a coup or whatever.
By OP’s logic, then, that’s good, and if they refuse they should become a non-citizen.
A spoken oath is a pretty ineffective enforcement mechanism, for what it’s worth.