I think the way blizzard handled defeating Sargeras was acceptable enough, how else are you going to defeat someone like that? It felt believable enough given the whole journey of the expansion, the titan artifacts etc, an impossible chance, made possible in an acceptable way.
The way they used the titans etc, what could have used more explanation however was us defeating Argus form. Were we enhanced by the titans and by how we were transported to the seat of the Pantheon? Also a bit dodgy was Aggramar's defeat.. was he weakened by the bosses we defeated earlier? We certainly weren't fighting his full form, yet it was him! so what were we fighting? His soul I guess, and we have no idea what that means in terms of power or defeatability, because it is so vague how can we say it was impossible for us to defeat them?
How we powered up:
1. Artifact weapons - we gained immense power from them
2. Netherlight Crucible - a fusion of a light and void godling to enhance
3. The Army of the Light
4. The Pillars of Creation
5. The Nightwell
6. The vindicaar seriously empowered.
7. Illidan
8. Eonar
9. Legendaries
Did we get any titanic or kaldorei power from the Tomb of Sargeras/Cathedral of Eternal Night.
Most of these weren't used inside Antorus, they got us to there, but we had the artefact weapons, legendaries and Eonar after her encounter to help the rest of the fight.
The thing is most of the bosses we defeated weren't necessarily aiding or boosting the power of the bosses, so it's not like Antorus was weakened the further we progressed and the more we fought.
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Ig uess we fought the titans' souls, and not them actually.. This is one of the reasons how and why the victory is believable enough.
The titans at the seat of the pantheon, their greatest seat of power are able to use their power to defeat Sargeras, or at least suck him back from Azeroth and somehow contain him. Not sure how that works, but he obviously couldn't use his body to destroy the seat, perhaps because they got his soul first.
It's acceptable enough if you ask me.