My point is that it's comparing apples and oranges. D&D happens to split their core material over three books. Your suggestion for 40k would require a "core book" - that requires a non-core book to play.
I know what your point is, but your point begs a question that still hasn't been answered: Why does this make a difference at all? Who cares that you have to buy a core book and a non-core book to play a game?
The terminology here being used is inaccurate and confusing the point.
There would not be a core book and a non-core book required to pay a game
There would be one universal core book/ruleset - that each game line would share - and a second "core" book for each game line, with the things specific/exclusive to that game line.
There would then be additional splatbooks for each game line, but each line and the splatbooks thereof, would be all fully cross-compatible, and designed as such.
Or that's my understanding.