My thought started out with Dumbledore, and how his supporters think he's the best thing since floating broomsticks, and how his anti-supporters think he's a barmy old nutter or worse. Anybody who's Anybody in wizarding society pays careful respect to Dumbledore unless they're one of the open opposition, but look who the people closest to him seem to be, judging by the members of the Order: the Weasleys, fugitives from the law, werewolves, Death Eater double agents, and Mundungus Fletcher. Does that look like the majority of the wizarding population actually puts their trust in Dumbledore? Um, no. I'd guess that the majority of the wizarding population is right where Az-the-Elder and I would be: hunkered down in the Wizarding equivalent of Switzerland, ready to come out after all the fighting's over and get on with our lives. (garnetdagger would pack me off there in an instant, and she'd pull a Need with a sword, only without the stab-through-heart bit. (Erm, for those who don't follow Lackey, she'd transfer her spirit into an already-magical sword and soul-bond with a lucky user.) )
Then I started thinking about the size of Hogwarts. It looks like there are relatively few students there. Like, perhaps 30 to a year in a House. There are seven years at Hogwarts, and four houses. That's 840 students, approximately. Now figure in another eleven years of wizarding kids that aren't at Hogwarts yet at the same rate, not counting the inevitable post-war baby boom that's going to be hitting Hogwarts when Ginny gets there. At eleven times 120, that's 1320 wizarding kids who'll be going through Hogwarts who have already been born, at the current rate. So. 2160 wizarding children under the age of majority in Wizarding Britan.
Now. What's a typical wizard family size? We've got the Malfoys, and I am guessing that Draco is an only child. Harry's an only child, and I think Hermione may be as well. We've got a couple sets of siblings around, and then we've got the Weasley clan. Is it safe to guess that wizarding family sizes may be, on average, around the same as Muggle family sizes? The average Muggle family hereabouts has around 2 kids for a 2 parent household. So double the number of kids about, and you've got their parents and probably some of the wizards of family-raising age who haven't had children to make up for the Weasleys. 4320 wizards. Double that again if you want to think about the usual wizard lifespan being longer than a Muggle lifespan... 8640.
9,000 people is a very small number. According to Ireland's census, a town of 1,500 is the dividing line between town and rural. If there were a wizarding population of only this size, there could only be maybe ten very small wizarding communities, discounting all the rest of the scattered wizards.
No, there have to be more wizards than that, and that means more children. The wizarding children have to be going somewhere else to learn their craft. My thought on the matter is this: the vast majority of the Traditional Old Blood are looking at this Wizarding Academy and saying "Pfa! Academy? I apprenticed, my father apprenticed, his father apprenticed, and if it was good enough for them, it's good enough for you, and stop whingeing about this Hogwarts letter. Hogwarts? Hogwash, I say!"
That's where the brains that have got to be in wizarding society are going. The Hogwarts students are quite probably the progressive, the rich, the Muggleborn, and the pretensious. Snape's peers are supervising their work and their journeymen, and the journeymen are bossing the apprentices around. Any kid too young to apprentice is probably still being chased after by the homemaker parent and/or nurse.
Hogwarts is an Ancient and Noble House, but it hasn't been around for any 2,000 years. Another five hundred years from now, more wizard-born wizards may go there, but for now, it's still experimental, and the Slytherin Problem (put all the ambitious ones in one house, and a few rotten ones can throw the whole barrel into suspicion) is proving out the "I told you so"s of the grans who won't hear of anything other than apprenticing the kids.