I think people don't realize why we're complaining about the new arc.
The new arc is not bad by any means, the problem is. After damn near 150+ chapters of mild or next to no progress with the two main characters. They place in a new character to rush development and I'm guessing there is a reason for this because like in most manga like this and once it draws on too long w no development eventually people stop reading since it never really gets anywhere.
But it's mostly like where was this like 100 chapters ago why bring it up now? That's the issue, where was this plot development when you know needed it like 100 chapters ago.
Still a good arc but you can see why some people don't like it, it's like sorta outta right field and shit to make drama between the two main characters but if we're being honest here Amano is still going to win regardless. So why even bother bring the secondary female character you know?