I don't think that's what they're talking about though, it's more like Diablo 3 being the same length but with different enemies, then we find a reason to fight Malthael, then another expansion has us finally take down Diablo. The idea is to make Diablo the big bad of a Diablo story instead of someone who gets taken out partway through. Look at D3 now - Diablo is just another sub boss you take down on the way to Malthael.