* I suspect that, as the marketing data indicated was likely, the minis sold very, very well. If the marketing data is correct, then the sale of books is 1/10th the sale of minis; the game becomes a means of selling minis, rather than the minis becoming an adjunct of the game.
While I don't necessarily disagree with your larger point, this does not logically follow from the observation that "gamers who buy minis spend 10x what gamers who don't buy minis spend."
More plausible, I think, is the explanation that some gamers are deeply committed to the game and some gamers are not. Those who are not committed to the game are unlikely to drop money on anything but the basics - a Player's Handbook and a few dice. They aren't going to be out buying minis.
Anyone who does buy minis is almost certain to be a committed gamer, and therefore willing to drop lots of money on the game. That doesn't mean all or even most of that money will be spent on minis. Until the last few years, I had bought maybe 2-3 minis in my entire life; but I had a heap of D&D books big enough to shingle a house with.