When Blizzard announced the Hellfire Citadel raid and Tanaan Jungle they said the same about it and the story of Warlords of Draenor, even though we were only at 2 raid tiers for that expansion.
This expansion started on the Broken Shore with the Tomb of Sargeras. It makes sense for it to end there. We'll kill Kil'jaeden after defeating the risen Avatar of Sargeras. WoD ended with us killing Archimonde, Legion will end with us killing Kil'jaeden. It's like poetry.
Who would we even face on Argus? We're not gonna face Sargeras, no way. And both Kil'jaeden and Archimonde are dead. Who would be a fitting endboss for Legion other than Kil'jaeden?
And in addition to that, Argus just has too much potential to only be a patch. It has to be its own expansion.