In WW1 Princes died in the trenches, in the War of the Roses the nobles exterminated themselves. In Rome patricians who wanted political power had to serve in the army. At Cannae both Consuls (POTUS and VP analogues) were present, one was KIA. Julius of impeccable lineage had no problem with standing in the front lines to inspire the troops.
Tech though began creating an easy out for upper types. Plenty of WW1 generals found no need to go to trenches, many stayed so far back they could barely hear the arty, aided and abetted by the static nature of the front and radios.
WW1 dead princes:
Prince Heinrich of Bavaria, Prince Friedrich Karl of Prussia and Prince Frederick John of Saxe-Meiningen, Belgium's Baudouin de Ligne as well as England's Prince Maurice of Battenberg.
Most of the cases you quote here are Major War (or worse, civil war in the case of the roses) issues, so most nobelity was involved on them, as their posts were in jeopardy. Most of traveller adventures are about pretty missions on wich hight nobles whouls be out of place (unless as patrons).
Hight nobles in 3I, as I understand it, are true administrators of large zones and larger populations, so they should be deserting ther posts if going on adventuring. So, if a player on a campaign I'm refereing comes to have too high SS, I assume it's a distant cousin of a hight noble, not a noble by himself.
Even if close kindsmen of hight nobles, I don't think they whould be allowed to go adventuring for fear of bein taken as hostages (look what happened when the grandson of the british queen was set to Afghanistan with his regiment and extrapolate it to 3I*).
That doesn't mean that a campaign may not be customized for hight nobles (or high rank military), either as political intrigue near Capital, as diplomats, or anything else a referee can think about (e.g. Arrival Vengeance, if you play it with the players as part of the senior staff).
*EDIT: After all, Afghanistan is a unconventional war, and a minor one for most people (hope no one takes offense, even if he/she disagrees in this point. There are no minor wars for those involved on them). I don't think the reaction whould have been the same if sent to a regular or major war (let's say, should he been sent with the BAOR to deffend Germany. Actually, his uncle fought on the Falklands and there seemed to be no worries about him to become a primary target or a hostage).
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