You can't advertise a big bad boss then only allow .0001% of players to get to it in the expansions time. Would be like selling a game about WW2, but you can only fight nazis if you completed the game 15+ times on the hardest difficulty. I could see that for heroic, as any rag-tag group will get it eventually, but not mythic.
I'm pretty sure Sargeras will feature in the lower difficulties, too, in the background. We know he does something to Azeroth after all, they have to have his model do it. My guess is that Sargeras either overtakes Argus so we fight a mini version of him, or he just decides to start casting instead of standing there. I kinda hope it's the former.
Here's my reasoning. Illidan should not be alive, yet he was brought back, twice, and he gets to be the jailor. He was brought back for bullshitty reasons as fan service. Having a bullshitty reason for fan service of why Sargeras has lost power (fighting the void for thousands of years) to a level where we can fight him isn't that far fetched.
Getting rid of Sargeras (for the foreseeable future) along with Arthas dying are the biggest milestones in this game for me when it comes to lore. The first time i heard Arthas was going to be a boss i felt that sounded dumb, we were just mere mortals. Now icecrown citadel and how they handled his fight and the cinematic are one of if not the best memory i have of this game, at least when it comes to lore. Back then i only played normal raids, and the notion of them making something even close to that cool a mythic only thing makes me sad for people who don't do mythic now, like i couldn't back then. This is what my original post was about.
It does feel a bit weird if we're killing the snake by slowly chopping the body strangling us rather than getting rid of the head (while Illidan has been telling us the whole time to go for the head). If the game would ask if i wanted to be the jailor of Sargeras at the cost of deleting my char i'd do it in an instant.
@Bjoramier of Lordaeron is actually right on the money with his lore knowledge. You should be thankful that he was able to set you straight. That axe actually drew blood from Sargeras. You know, the dark titan?
Infused with power directly from Cenarius.
Could be more powerful could be less who knows.
Fact is though it's the only known weapon and Brox is the only known mortal to directly inflict a wound on Sargeras.
So @Bjoramier of Lordaeron is pretty much right. Can't really add anything to what he said other than my own lore knowledge.
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Nobody focused him down. They were trying to close a portal.
Brox jumped through it to give them more time.
https://wow.gamepedia.com/Broxigar
Broxigar lore if you are interested.
I think people are jumping the gun about fighting Sargeras directly. As of the time of this writing, he has no in-game model and might just appear in the cinematic
But yeah, if he's mythic only I do think thats a shame, sort of like the Cho'gall thing in Highmaul
I personally think it'll be similar to Gul'dan where Sargeras interacts during the back end of the fight while being imprisoned when it ends you get the cinematic, mythic only phase will be spending some time actually fighting to subdue / weaken him slightly to imprison him, similar to the demon within phase which isn't some crazy amazing lore points that LFR Hunter Bówjóbxo can't see.