Ovinomancer
No flips for you!
I was thinking that one key factor in helping to answer this question is the number of classed characters in the world, or at least the "known world", for reasons I'll explain below.
There was another thread devoted to this topic about a month ago, How Many Adventurers are in Your World? The answers varied from "no more than a dozen people in each class in the world" to something along the lines of 2/3 of the population having more than one HD (or level).
A proportion along the lines of 1 out of every 100 seems like a reasonable suggestion toward the middle of the curve, and it roughly corresponds to the size of non-productive population in pre-industrial society (on the conservative side). DM's answers to this question will obviously vary, but for two key reasons, I don't think this number will be significantly lower than this. One, even if the PCs are close to unique, the number of monsters they encounter and are aware of in the course of their career will likely have the strength to overwhelm a civilized society that has significantly fewer classed characters. And two, even if they don't, even fairly low level PCs will (even if Providence has now established Bounded Accuracy). So the critical mass is probably somewhere in the ballpark of 1%.
How many classed people would that mean in absolute terms? There is a long-established precedent for viewing the standard fantasy society (like Faerun) as a representation of 14th century Europe. The European population in the 14th century was roughly 75 million. It was slightly bigger in China, so 80 million seems like a good ballpark estimate for a fantasy realm that metonymically equates itself with the whole world. This would mean that in a Faerun, or a Kara-Tur, there would be roughly 800,000 classed NPCs. If we go with another conservative estimate, and assume that 90% of these would be fighters and rogues (and maybe barbarians, depending on where you are), that leaves 80,000 characters of other types, predominately spellcasters.
Now, let's say you are church administrator or a shih - a Confucian-type scholar-official. You belong to a body that possesses a virtual monopoly on literacy, education, and the diffusion of information. You are at least vaguely informed about the key things going on in your realm. One of the main things you are going to worry about is how to manage that 80,000 (assuming the fighters are somehow being managed by various military establishments, and the rogues can be kept on the margins, possibly to police themselves). The idea that 80,000 people, with significantly greater powers than the average person, and with the potential to become truly superhuman, are just being born randomly, and continue randomly running around, murder-hoboing their way through dungeons, the wilds, and ultimately, cities strains credulity. That 80,000 is going to be one of your overriding concerns. Because any society with a pyramidal structure, where the vast majority are primary producers, is going to have to regulate those with unusual powers to ensure that the hierarchy is perpetuated, that they remain onside with those who govern (or become those who govern). And to ensure that factional strife between some of these people (who likely have pretty pathological levels of self-esteem if not tightly regulated) doesn't lead to social breakdown in a civilization which is usually just a few failed harvests away from collapse. And to ensure that the majority of the population sees the supernatural abilities of this 80,000 (or at least a majority of this 80,000) as legitimate and engaged in upholding the social order.
So to this end, you will focus a lot of effort on tracking where and when such people are born, on making sure they get proper training, on ordering them institutionally, and on establishing a system of sanctions in case any one of them is using their powers in the "wrong" way, or simply derives their powers from a source that's impossible to control or define. Obviously, clerics ordered by temple and domain, and wizards ordered by schools (which administer access to spellbooks, among other things) are probably going to be the more regulated classes (and, for all of the above reasons, they will be managed as classes of people). Sorcerers, warlocks, and druids will be on the other end of the spectrum, and less class-like - maybe. Because if they are controlled against their will (or even exterminated), they will likely develop some sort of organization to recognize their own members, and to offer some sort of resistance, to avoid being picked off individually. And if they don't do this, there is every reason to expect that institutions that claim a monopoly on knowledge, spirituality, and legitimation will crush them completely in the medium-to-long run.
Now, the number of such gifted people might be lower. The political and religious powers that be might be weaker than in the hypothetical worlds described (given the prevalence of polytheism in the typical D&D fantasy environment, and the absence of a Chinese-type centralized empire, may make political control less effective. But whatever the type of political organization, control over supernatural powers and people will be an overriding concern, societies will invest significant resources in regulating such people, and therefore, some sort of class-type organization, with distinct and non-transferable powers, rituals, ethos, and a particular place in the social hierarchy will probably form over a period of time.
That's not RAW, but it would make a fine setting.
In my campaign, I have no idea how many classed people there are. Doesn't matter. Most people have more than one HD as NPCs, and a number are quite powerful without being classed at all. So any classed abilities will disappear into the general morass of similar unclassed NPC abilties.
Again, this is an issue that's highly setting dependent, and is not an inescapable conclusion of the rulesets. Your example above is just you coming up with a setting and, in no way, is an example of how all settings would or could work.