The Primarchs are to the Emperor as the 12 Apostles are to Jesus. They have been dead or missing for centuries. While there are legends that some of them might return, I would think if they didn't return for the 13th Black Crusade, the BG mega battle would have to have a very high octane story line in order for it to be vaguely believable.
I play with little plastic men and cardboard armored robots that go pew pew. I have little issues suspending disbelief.
Different people play this hobby for different reasons. They enjoy different things. One aspect is modeling. The game is the better for fully painted armies and well put together scratch builds. Thus, we have rules about hamster ball and toilet tube models, and inspections to confirm that the modeling is worthy.
Primarchs in a modern era game? From a fluff perspective, at least as I see it, this is roughly the equivalent of hamster ball and toilet tube.
I care about all three elements, about the wargaming, the modeling, and the story line. Not everyone is going to be equally good at all three, but it would be nice if we could at least try to respect each other's interests.