Sheevism didn't triumph; Sheevites just got louder and clung to even more ridiculous methods of argument such as the whole "balance of the Force" arc as if Palpatine was exclusively responsible for the shift with a Force push. Ridiculous.
Balance of the Force is intrinsically linked to acts of individuals across the galaxy because it permeates all of space and binds everything within it. However, living beings have a far greater impact on the balance of the Force than inanimate objects. When corruption, perversion, conflict, oppression and killings become widespread, the Force will loose its balance and tilt toward Darkness. Sith are also known to perform evil experiments which may nudge the Force further into Darkness. Due to these factors, the Jedi attempt to resolve conflicts, preach morality and eliminate Sith. A source named Star Wars: Force and Destiny offers a decent explanation of the mechanics of the balance of the Force.
However, the desperation of Sheevites reached to an extent that they began to argue that Darth(s) Plagueis and Sidious were influencing the balance of the Force with their Force powers; as if their rituals and evil experiments are a demonstration of their raw power, and all else was irrelevant. I wonder why they were not tearing planets apart with a gesture then.
Over-the-top ridiculous arguments in short.
BioWare simply ignored the whole "balance of the Force" arc in its storytelling. Drew Karpyshyn touched this subject briefly in the Revan novel, and full stop. The author made it obvious that Vitiate was the source of Darkness and agents of Light were emerging to oppose him from time-to-time (Revan being one of them).
Vitiate has a history of performing some of the most evil experiments (ever) and common sense dictates that such experiments would have impacted the balance of the Force. And then there are a number of other factors to consider.
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When I compare the characters, I am not focusing on the wider implications of their deeds, I concentrate on their raw power, showings and implied levels of strength.