well, that's like.. you opinion man. You know they can only push so far making Med 3, Shogun 3, Rome 3, Empire 2 and so on... anyway. Line battles are not the sole defining mechanic of Total War and are present in most so called "strategy games" (most of them are actually tactical games with some front line resource gathering trowed in). Total War most striking characteristic is the economically successful combination of a rather shallow turn based "strategy" (which just keep getting shallower in every subsequent game) combined with very detailed and engaging real time tactical battles.
In fact it took them a good deal of time and coding to get proper battle lines in Rome 2 , it was just a bar brawl at first. Also we have things like Total War Battles, which have no a single linear battle-order nowhere in sight, more like Civ V chess, yet is called Total War, and not to mention their new Facebook Total War or whatever crap they are up to now. The point is, Total War is whatever CA says it is, not you or me, sadly.
Also a lot of WH40k battles look straight out from World War 1, especially IG battles, and guess what mod has been done for empire? (or was it Napoleon?).
On the other hand, again, you are entitled to your own opinion, but Empire at war was a good game and worked very good from a gameplay and economic point of view. On the matter at hand, the gameplay, the space battles did far better than the land ones yes, but there's no reason no to mix them in another hypothetically new game, as long both of them are made properly.
Simply try to imagine this: A 40K game like Empire at war, with it's Battlefleet Gothic like space battles, but replacing each planet with a map like the one from Dark Crusade and of course having land battles like the ones from Dawn of War. It is so hard to picture it?