Actually, you did not read my post upthread carefully. The quote above is NOT correct. There may be any number of Knights, Barons, (or even higher) in any given system. It is that there is only one Landed Knight, and one Landed Baron in the particular case you described above (who each have an assigned Imperial administrative function). There may potentially be an unspecified number of Ceremonial (Administrative) Nobles of any rank, and a likewise unspecified number of Honor Nobles of any rank. Only the Imperially-appointed Landed Nobility for a given world are specified in the Noble Extension in T5 (and on Traveller Map).
No, I didn't read your post carefully, nor was I responding to your post specifically.
You are making the error that Nobility = World Ruler (and/or other government official). This was my point: Nobility is a Social Class distinction with privileges (nothing more). A Noble need not have any official function. Nobility of the appropriate rank does, however, qualify you told hold certain posts (but does not guarantee or specify any - such an appointment is the pleasure of the Emperor).
The scale is still way, way off. Orders of magnitude off. Historically, a Baron didn't rule a large area at all. A few thousand acres, enough for a decent sized village. A knight in the post-Hastings arrangement of England would be granted a
hide, which was nominally 120 acres (some would be granted multiple hides). For comparison, a peasant farmer would need a lease of about 15 acres to support a family. Knights were subordinate to Barons, a few dozen for weak Barons, hundreds for strong.
A Count rules an area corresponding to a County; a Prince rules a Principality; a (Grand) Duke rules a (Grand) Duchy. A March was a County in a border area that required some semblance of a standing defense force above the usual assortment of Barons and knights. The Duchy or Grand Duchy would be a tiny or small country, respectively. Think Liechtenstein (actually a Barony) or Luxemburg (the world's last remaining sovereign Grand Duchy). Monaco is a Principality, but it is an oddball case (a city-state that once wielded far greater influence than it's current money-laundering function). Technically, Wales is a Principality even though it is a bit larger than most. It had been a Kingdom but was demoted after conquest.
The Kingdoms of France and Spain were the largest polities in Europe for a very long time. They overshadowed everybody else.
Actual rulership of a planet with any decent sized population can't devolve to the rank of Baron. There is simply too much power at stake. The way a feudal system works is to keep the fiefs small and the fiefholders divided by their own interests and loyal to the Crown. Perhaps a minor territorial ruler on a non-unified planet might be considered a Baron, with larger countries' rulers placed higher in the rankings. Most modern Kingdoms would be demoted to Principalities. The biggest territories, like the USA and Chi-Coms, might be considered Duchies.
The model for the Imperium "ruling the space between the stars" is a clever idea, but entirely unworkable. Interstellar trade would be only the tiniest fraction of a world's GDP. If Imperial Nobles could only tax that trade they would be individually wealthy beyond our grasp, but too poor to actually build and maintain defense fleets needed. Even on a unitary world, Knight, Baron, Viscount, Count, and Marquis would all be fractional subordinate landholdings. Only Princes and Dukes would have whole worlds (rather small ones at that).
For a position that rules an entire settled and developed planet, you might have Grand Princes, Grand Dukes. Kings would only be planets or systems that were independent from the interstellar Imperium/Republic/Federation polity. Archprinces and Archdukes would have authority over multiple subordinate Grands, but would not actually hold more than one planet. Unless the Emperor is very weak, perhaps an elected position drawn from a pool of Arches ruling multiple main-pop worlds.
Dune had it kinda correct. Main population worlds were Duchies (Herbert didn't develop a complicated system of Grands and Arches). Harkonnen was an exceptional Barony in terms of power. Dune itself would be considered a Palatinate, owned by the Seat of the Emperor, and managed as the Emperor assigns.
Therefore, when the CharGen system is used to generate a PC or NPC and a Noble-SOC is generated, there is no specification whatsoever that that character is THE Landed Noble of the homeworld (unless the referee chooses to specify them as such). But that would severely limit the kind of campaign that the character could participate in, as such a Noble would have official responsibilities.
Now to be fair of course, this is the IMTU forum, so if you want it to work differently for your campaign, go for it. But the thread is about Ageing by TL, and Spinward Flow raised the issue of Imperial Nobility in the Third Imperium in that context.
Yes, nobility would have casual access to anagathics and such, with title holders probably reigning a couple hundred years. They would marry and have families rather late in that cycle, to avoid having many generations overlapping and vying for the seat. Pity Prince Charles, who shall likely be passed over for William. There are plenty of distopian stories of advancement through assassination when the stakes are so high. "Simply unfair. It's my turn to be King. Mummy must hand over now. We should arrange something..."