Why did this wisdom become conventional? Do you spend more on minis than on RPG product? Do most of your gaming friends? What about all those people who refuse to buy D&D minis?
I can only speak for what I have seen locally.
D&D book purchases are sparatic. After the initial books, anything else may or may not be bought depending on the nature of the book. Not everyone buys all the settings related books etc.
So you have the $100 purchase of books as a one time thing for DMs/groups and the PHB purchase.
Most of those people would buy the same number of minis if not more dollars wise for RPG if they use minis.
Some buy much more for RPG use, than they spend on the books.
Also you have the hardcore minis players/collectors. While not keeping all the minis, they buy packs to get the chase rares, or figures that they most like such as named NPCs. The minis players on average would play sealed games without prize support, and constantly be buying minis after the RPG players have stopped. Usually playing several games can exhaust a case of boosters quickly with 4 people as that is only 3 games.
Then you have kids who may want minis as toys that play neither game and the odds of buying mixed genre's such as Superheros from Clix games,, monsters form DDM and tanks/etc from A&A are likely.
So it is not unheard of for people to spend more on the accessories for a game, than the game itself, because most times you only need on DMG, unless you are buying them for gifts or to loan out.
And what person ever really gets rid of the want for little plastic army men that they played with as a child?