Yes, the Scepter book said "she would be his" but the entire basis of Legion was still an apocalypse plot per every other word of the Legion's in the same expansion. Sargeras can be possessive and also destructive, he's generally considered fucking insane by his kin. That's if we're even taking the artifact research as completely accurate considering it isn't Doylist. It's order hall researchers.
Again, if we work backwards from the concept of us not stopping the other plans intended, Azeroth would have already been fucked on a planetary level and not just a soul.
Chronicle on the other hand was at one time intended to be and released almost simultaneously with Legion. A whole 5 month gap.
Argus isn't headcanon, it's an immutable fact. It's the only established exception outside the Burning Crusade's intentions. Turaylon explicitly states they're using the soul for regeneration. Other fucked up worlds exist but they're generally staging grounds for armies, none to our knowledge use world souls.
I'm not mad, it's par for the course for Blizzard writing. Sargeras had barely any time to do anything, let alone know for certain where he was stabbing. He almost killed the planet already. A few seconds more and he would have. And even then, if you wanna go with the corruption angle, the sword was indeed corrupting the land before we sacrificed the artifacts - which infers he was just aiming at the world soul. Nothing more complex than that.
If this wasn't just an easy mystery box to open, it would've been easy for Thrall to say "I heard a voice call out. Could it be Azeroth...?" at the Amirdrassil epilogue or something. Instead they're burying the lede and people are overanalyzing as always.
For that matter, if Sargeras is just the green pallete swap of the Jailer and the Void and has the same intentions, does nobody think that's kind of boring? Instead of a scared and nihilistic madman that would wipe everything to avoid a bad fate, he just has the same plan as every other villain? Chronicle would be even more worthless, and the Titans would just be stupid and wrong if it's their POV.