Seriously, if they were the main antagonists then the main plot of the story regarding the Legendary Pokemon and Antagonists would mesh better together, that's not to say that SwSh would suddenly become good games mind you but at the very least it'd be a better plot;

> Sonia
Sonia's character arc is completed through the conflict that arises between her true history of Galar research and the brothers' desire to keep things under the status quo, they would likely learn of Sonia's intentions and actively attempt to stop her, this is somewhat shown in the Post-Game plot with them using an undercover agent to spy on her and steal wishing stars.
> Zacian & Zamazenta
The conflict between the Legendary Wolves and the descendants of the twin kings is a more interesting and direct conflict than that of the Darkest Day, plus the history of the Darkest Day already tells us that the Legendary Wolves were able to overcome Eternatus in the ancient past. Having the conflict instead be between Zacian & Zamazenta's true history and the history that was fabricated by the ancestors of Sordward & Shielbert would make for a better duality and would allow for more direct focus in regards to the Legendary Wolves and how they impact the antagonists.
> Team Yell
Team Yell in contrast to the followers of the royal lineage makes for a good comparison, both groups adore their respective idol (Marnie and Sordward & Shielbert respectively) and while Team Yell comes off more light-hearted and jovial in nature, the followers take on a more nefarious and ill-willed variant, it would make for a good contrast between the supportive Team Yell and the antagonistic followers.
> Bede & Oleana
Put simply both character’s personalities work well in regards to Sordward and Shielbert, Bede being raised under their guidance would be an example of fanaticism to an extreme while Oleana’s general prim and proper personality towards Chairman Rose seems similar to how some regard people of regal descent.