So ... nobles aren't all identical but they use the same stat block because that's the default entry? So every male in the U.S. is 5'9" and 197.9 pounds (thanks google) because that's the average? You were the one making the claim that nobles would be easier to kill because your wizard has 22 HP and the noble only has 9, not me.
Nobles were rarely assassinated because personal combat capability of any individual is not particularly relevant when it comes to the functioning of a nation. Either every noble is the same or they are not. Either individual prowess at combat matters for most nobles or it does not. Choose a lane. Local warlords may need fighting prowess (whether martial or arcane), rulers of larger nations do not.
As far as the rest, I have no idea where you're coming from.
If we assume that the individual combat expertise of a noble matters, there's no reason to believe that any PC class has a particular advantage. Back when leaders did go to battle*, they were there primarily as moral support and to inspire the soldiers. I think in many cases the soldier is going to be more motivated seeing a leader that's a fighter than someone running around twiddling their fingers.
But it's not particularly relevant. The power of the state does not lie in an individual except for the fact that an individual is a figurehead and symbol. People lined up for blocks to see Queen Elizabeth's coffin because she stood for something greater than the woman. She was a symbol, an idealized embodiment of national pride. She was beloved and, last I checked, she had nothing that would qualify her for combat duty even if she did join the armed forces for a period of time.
P.S. I was, again, using champion as a generic term as in the subclass of fighters and an off the top of my head example of a fighter not trying to one-up your wizard example. You're picking on minutiae instead of the content of the posting.
*Which died out by around 1500ish. Even before then most were never truly on the front lines, or if they were they were surrounded by the best troops available.