You raise a good point about the royalties Creative Assembly pays to Games Workshop, but I think you underestimate the importance of DLCs. Due to how grossly disproportional the ratio between price and cost (in favour of the former, of course) is, the trend of the industry is to move towards small digital packs, which offer much larger opportunities for net profit than the nominally more expensive tent-pole titles. Electronic Arts has already been gaining
twice as much from DLCs than the main product since 2015, so I'm sure that SEGA and every other video-game flagship will try to imitate their example. The retention rates of Warhammer help a lot in this regard, not to mention the fact that its fandom is much more willing to spend her money on these digital packs.
Moreover, they are more easily justified and sold than their historical counterparts. Can you imagine
Armenian archers from Empire or the snake catapults from Rome II trying to compete with unique lords, flying dragons and their extravagant models? Finally, although it is true that Warhammer's II performance was commercially disappointing, in terms of pre-orders, the sales probably recovered, as gradually more potential customers immersed themselves into it. Pre-order purchases matter much more than the rest, but when all these factors, when taken together, probably outweigh the advantages of Three Kingdoms and future historical titles.
Still, in my opinion, the example of Troy is the most convincing argument. Not only did Creative Assembly add superheroes and Minotaur furries, but the company didn't even bother to appease the more conservative crowd with a more historical mode. Presumably because Romance crushed Records in popularity. To sum up, the player-base has changed radically, in a similar manner it had already evolved, during the transition from Medieval II to Empire.