First, the disclaimer: I'm not asserting this as true; without evidence one way or the other, it's just speculation. This is also specifically about what happened during Kirby's tenure; I'm thinking about the historical rather than the present.
So, I was thinking.
I recall seeing it said on a number of occasions that GW cancelled the "specialist games" because they felt they were competing with themselves. The idea was that Bob will spend X amount of money on GW product, and that the specialist games were competing with 40k and Fantasy for that money. By removing the specialist games, GW allegedly thought they would still make the same money from Bob without having to support numerous games and model ranges, thus cutting their expenses.
Now, I can't find an actual source for this, so this theory might just be a load of internet nonsense. However, lacking sales figures or an official statement from GW, I can't prove that the specialist games were choked out and then cancelled because of poor sales either. Given some statements made by Kirby, such as the primary hobby being buying models, and given the stubbornly enduring popularity of games like Blood Bowl, the idea seems plausible.
Let's assume for the moment that it's true, because this is an if-then statement. Assume for the moment that GW really did think that customers would continue to spend about the same amount of money on the hobby regardless, and that they could increase profits by removing the costs of other games and model ranges. Assume GW wanted to narrow the focus of its customers by getting them to just play 40k, instead of 40k, Necromunda, Battlefleet Gothic, etc.
If that's true (and again, I'm not aware of actual evidence one way or the other), what if they wanted to narrow it further?
Tinfoil hats are a reasonable option at this point.
So you've got this game, and the game has a bunch of model lines, and supporting them all is kind of expensive. You could save a lot of money if you can convince as many of your customers as possible to all collect the same army. It's not great for the game, but you're really a model company not a game company anyway. You can keep the other armies in nominal production, with occasional, sporadic support, but you can save a lot of money if you only need to focus on a single model range, while concentrating your sales there.
What if GW made a sustained, concerted effort to get as many players as possible to collect Space Marines, because only really supporting one army lowers production costs while focusing sales on the kits you do produce.
If a sample of 10 players each play a different army, then you need to produce 10 different kits in order to get 10 sales. But if you can get 7 of those players to all play Space Marines, you can sell one kit, or one kit with some variant parts, 7 times. At that point maybe you don't even need to care about the other 3 players.
Now, again, I don't know that this is true. It's just a thought that I had. I'm also not suggesting that your love of Space Marines is somehow a result of insidious GW mind control. I would suggest that 40k players who aren't interested in Space Marines are more likely to wander off and play other games instead, because historically they have not exactly been encouraged to stay. I'm also not saying that, if this is what happened, it was in any way a good plan; I'm saying they might have thought it was.
It does seem like GW's marketing over the years has been aggressively focused on Space Marines. It also seems like they would benefit financially, at least in the short term, if most players bought Space Marines instead of other armies.
I dunno. It seems plausible. What do you think?