Agreement is good.
c) the rationale is, of course, that the BBEG has been training these guys for years and-or they've gained their levels by raiding the countryside and generally being bad guys...
Yes, but to me, that rational does not explain the minions being more than about 4th level. You don't get to 10th level in the lifetime of a mortal by training at arms and riding around occassionally killing commonners that resist tax collection.
...or they're just mercenaries and the BBEG pays 'em really well!
Which again begs the question what is the genesis of 10th level characters in your campaign world. If these are 10th level mercs, then they've had adventures every bit as impressive and world shaping as those that the PC's had reaching 10th level. Maybe it was stretched out over a slightly longer period of time, but these would certainly not be nameless mooks. These would be individuals whose blades had each slaughtered the forces of good in the hundreds. They would be names uttered only in scared whispers. Prayers would be made for deliverance from each of them in temples across wide swaths of the campaign world, and these individuals would rightly think that thier larger than life lives earned them more than being ordinary if well paid guards. They'd want dominion and authority that corresponded to their outsized, supersized, superheroic abilitiy. They'd want palaces and haram girls and all the stuff and benefits that would go with their extraordinary pay.
And this also raises the issue of just how many high level characters a campaign world can support, given that as far as I am able to tell, the only really efficient way to gain experience is to engage things in mortal combat. Each high level character represents the death of hundreds of foes. A small army of high level characters represents the denuding of the countryside of life. If you did a computer simulation where you filled the world with 1st level characters and set them to fight each other, it would become quite clear quickly that either the population has to crash or few can reach high level.
This raises the issue of just how effective training can be. If the PC's decide to take two years off for training, work and study, how many levels can they gain if any? The answer better be close to zero if the idea of having adventures is to make sense at all.
d) I'll disagree here. Consider this: right now your PCs are out in the field gettin' it done, and in a few years they'll be 10th, 15th, 20th level. Then they'll retire from adventuring...and in 25 years what will they be? Yep, those very same stay-at-home high-levels you say you don't like. In other words, the stay-at-home high-level nobles and clerics and wizards of today are just yesterday's field adventurers enjoying a well-earned retirement.
Well, see the above argument against the existance of large numbers of people with the same sort of career as the PC's. I mean, how many people can save the town from a once in a decade tragedy each year? How much undiscovered treasure is there out there if we must assume that people discover as much as the PC's do several times as year? How many dungeons permeate the environment? Are there really 20 or 30 times as many dungeons on your map as get explored by the PC's? Where do they all come from? Is there some sort of seed that grows into a 200 year old dungeon in a few years time? Did each of those high level characters dispatch as many high level characters during their careers as the PC's dispatched? But now we are imagining an ecosystem that makes no sense whatsoever, since its all predators. Each PC must kill more than 4 characters of his own level in order to advance a level (and the ratio was MUCH MUCH worse in orlder editions). There's just no way to have a pyramid that extends up very high because the amount of violence required to get to high level implies the deaths of millions of 1st level characters. The only way to get it to extend much above 1st level is to assume that you can get roughly to level X by training. But if you can get to 20th level by training and 1st level is indeed a schlob, then why not get to level X via a training montage before going adventuring?
Finally, I've gotten characters up to high level and 'retired' them to be lords and leaders of the campaign world. However, I've played the 'retired adventurer who is now a lord' game as well, and it doesn't work like you describe. When your serfs are endangered, you go get the armor out of the closet and you go kill some orcs. Sure, you probably take along some henchmen and mercs so that they get some experience from watching you do most of the work, but you NEVER EVER spend good gold to hire a bunch of 1st level characters whom you don't even know to solve a problem you could more easily solve yourself without breaking a sweat. Sometimes you have to send the henchmen and retainers out alone if you are very busy with something and you are confident through scrying or whatever that the challenges are within their abilities, but adventurers don't end up existing as part of your economy. Nor do you allow strangers to plunder dungeons in your land and rob the riches thereof. You make every effort to find every dungeon, research every legend, and track down every cave and abandoned ruin so that if there is anything down there worth utilizing, you do so. Any large building that becomes abandoned upon someones death without a clear heir becomes the property of the king, and as explored and razed if necessary. You make sure everyone in the kingdom knows that if they stumble on something of archaelogical significance, that they are to report it immediately to someone in your chain of command for a nice reward and you deal harshly with peasants who seeking their own fortune go digging around in the Dungeon of Xyzzy the Mad.
In otherwords, the world filled with retired adventurers doesn't look like the stock D&D world, unless you assume that the good guys are absolutely as stupid and lazy as the archvillains of action movies. And when you see a world filled with retired high level adventurers (FR I'm looking squarely at you) that claims to work just like stock D&D, then you know its a silly gamist world where the demographics just work by DM fiat in order to create the gamist world where the PC's are forever some NPC's errand boys that the creator enjoyed.