Argus the unmaker might not even be the last boss in the raid, they could be holding another 1-2 bosses back for all we know. It's something they can easily patch in later on. I don't think they've ever done that in the past, but they've done a lot of new stuff in this expansion so I can see it happening. ( I do think this was done with Ra'den)
The only way I can see Sargeras surviving / not being the final boss is if they've made a the pawn of the void, which in itself would be a waste of his character. They've previously said that N'zoth was strong enough to corrupt Sargeras, so if N'zoth had the ability to reach that far then that's the only solution.
I am seeing a lot of void influence in the raid itself so that seems to be the only possible solution. Guess we'll just have to wait and see who's right, I assume we'll receive more info this month.