They could, but a LOT of factions would need to be gone.
-Ministorum and as such, their priests.
-Sisters of Battle.
-Grey Knights.
-Inquisition.
Plus
-Some Astartes chapters, like the Black Templars.
-Half of the Guard and the Navy.
-Maybe some Admech.
I think it would cut into their sales, but we all know GW is dumb, I mean, they did it with Fantasy. They may roll all custodes/Gk/other marines into Spessh Marynes, with armour made out of Emperorium and rename Earth as Emperoron or Emperorworld.