One problem was their clarification that there is but one Burning Legion, akin to the forces of Apokalips in the DC universe. While this can work as it does in DC, WoW encounters the problem that in theory the Burning Legion should never, ever be beaten if they can pool forces from [infinite] alternative realities.
They stated the Archimonde we fight at the end of WOD is the same as the one we also saw at Hyjal. Therefore Mannoroth is the same, as would be any other demon present [Archi trash, etc.] Any demon we kill in WOD returns to the Twisting Nether so they can reconstitute and return - as explained in the Chronicle [also why pre-fall Sargeras created Mardun as the prison world to stop this constant respawning of demons]. However primary universe A already has a Socrethar wandering around a floating rock in Netherstorm. That plot twist where he declared that name in the SMV quest was likely done in fan service to TBC players - I certainly got that feeling from it. [Likewise the inversion of the Blackhand/Ogrim story in Talador was similar fan service].
The problem however is that our defeat of Socrethar outside Shattrath wouldn't permanently kill him, since
he's a demon by that point, so he would eventually return. Even though we encounter his spirit in HFC and defeat him again his dying words are "The Nether, it consumes me!" indicating he's returning to the Nether to regenerate.
While it could be argued that perhaps Socrethar A is originally Socrethar B and once he returns he comes to Universe A, the fact they originate from an exarch that isn't a demon means each universe can likely have an Exarch Othaar. So every Exarch Othaar can then potentially become a Socrethar, permitting probability that not every Othaar will, and with no limit on universes [Kairozdormu line about "an infinite number of worlds" during his Ring chain battle] then the Burning legion shouldn't really ever be stopped if they manage to actually invade in full force. Even if they can't, unless we can kill demons in the Nether, it's just a stalling tactic until they finally get everything ready to drop a hammer down with the force of an 'infinite mass punch'.
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He portals in resources just as you set up your garrison as a one time thing.
He does this for perhaps a minute tops due to the power it needs.
The portals in Ashran are simply for player convenience and don't actually exist story wise.