They won't completely screw up the game. 40k is worth too much money and GW is doing absolutely fantastic financially now. They won't kick over the apple cart.
This has been my working theory. Basically, that there is no compelling reason to shake sh....things up, because 40k is the breadwinner. But I do have a certain level of fear that GW's internals are suggesting that they lose a certain number of players to simpler games, or that there is a prohibitive barrier to entry, etc...
Which is not to suggest that I'm averse to a shake-up or a simplification. The 40k rules are tough to get one's head around (I'm still terrible), and there is an almost perverse emphasis on knowing the rules that belong to each individual codex/expansion. I do think it would be of benefit to the game if it were possible to be remotely competitive (not in the sense of "tournament-competitive", but rather in the sense of being able to play a tactically-solid game) without having to have memorised your opponent's codex (or, as is increasingly common, codices...).
This is a dumb-ass example, because honestly I should have known better, but the first game of the first 40k tournament I played in, I had no idea what a Grav Centurion was. Therefore, I had no idea how it was going to fu...mess me up, or how to counter it. I don't count this as a mega-beef, because the Grav rule is in the book. It's more that if you haven't faced it before, you have no intuitive sense of what it "means".
And this is the thing - the main problem with 40k is that you need to have played a bunch of 40k to be able to play it properly. And I don't know how to solve that "issue", but I certainly don't trust GW to be able to do it well...