To those of you that don't understand why people don't like this arc:
Having a focus on different characters isn't the issue here. The problem stems from those particular characters and the situation they are in.
First characters, the heroes point of view is ... downright stupid. One view the world he lives in as a video game the other thinks discrimination is just going away by magic in a hundred year and therefore feels justified in enslaving them. This is beyond stupidity ... buuut that's kind of the point, having those heroes a warped view of the world will make for interesting conflicts later.
So far this series relied on having interesting characters so having those taking the focus feels like a break in how the story is being told. You could offset that with the war however. Conflict between the demons could be interesting, however there is issues in the reason for the conflict.
Why are we fighting ? Well ... we were told demons is bad. And ... that's it. That's the reason.
What are the stakes of the fight then ? Well ... if the heroes win they gain a stronghold in their fight against demons, but if they lose well ... nothing happen ? The status quo is maintained.
So we end up with un-engaging characters with no reason to fight nor stakes. This current arc has everything against it. And this is why so many people don't like it.
Comparison was made with other stories saying "well it happen in other stories that there is focus on different characters" Sure, but in those cases the side stories are interesting. At the end of the day, stories are meant to entertain, and this current one do not. Because we are totally detached from those characters, their reason to acts.
And if you open a parenthesis in your story and its not entertaining well ... your readers will be upset, that's normal.