I think the flaw in your argument is that the specialist games studio and the mainline studio are two very different teams with very different goals. 40k is the BIG name, even more so nowadays with it breaking out in the mainstream, so it makes sense for them to grab as many players as possible now the spotlight is on them.
Meanwhile SGS's job is to keep the long term fans and the more invested fans happy (and buying). So they bring out more granular, nostalgia driven, expansive games, like Horus Heresy, Old world, Bloodbowl, Necromunda etc. Not everything in the company needs to follow the 40k model; and probably SGS is better off if it doesn't. (Evidenced by the fact most of their projects are such runaway successes they have trouble keeping kits in stock).