Here's why:
1.Okay, so during the Q&A today when Ion mentioned going to Argus in 7.3, Lore said something along the lines of "now that we've fought the legion for the millionth time, it'd make sense to be a little more successful this time". You can watch the Q&A and see that for yourself.
1A.Why am I saying this? Because killing the leader/creator of the burning legion and the guy who killed all of the Titans isn't "a little more successful" it'd be completely successful. Now obviously he wouldn't want to reveal that we'd be facing Sargeras but there are other things that make me think we won't and it just doesn't seem to match up.
2. We fight the Fallen Avatar of Sargeras in the tomb. This is the Avatar that Aegwynn fought. This Avatar was only a small small margin of Sargeras' true power. Sure, we can beat it. But it wouldn't be a raid boss if it wasn't a powerful creature/being for us to take care of. So if the Avatar is along those lines of power (and it's been imprisoned for a looong time) there's no way we could face off against Sargeras just yet.
3. A lot of people say we will use our artifacts and "absorb" their power. That doesn't make sense especially when leveling through legion when it's over considering you can still obtain the artifacts when the expansion does end. Not only that, but there are some weapons that aren't "all powerful" and are better known for the person who wielded it previously. It just doesn't make sense.
4. We also have no idea if Sarg is even on Argus. We know that KJ made it like his own kingdom, but we have absolutely no idea of Sargeras is even there. He's a gigantic being that slices planets in half, who knows what he's doing?
Thoughts? Opinions? I could be wrong obviously, just curious to see what others think.