KnY was pretty mediocre last I read. I do agree art was sub par as well. It's popularity is clearly from the anime. It's a much better product than the Manga.
I wouldn't worry about it though. Sales of KnY from last year to this year was reduced by about 50 million volumes. If it continues dropping year after year, it'll fade away soon. Or it gets a bump again with the next season.
OP has been steady mainly through the force of its current content. The closer we head to the ending, we should see an increase otherwise that's when I'd be worried and sad if sales dip. And depending on the success of its other forms of media, it'll surely increase even higher. It's an amazing accomplishment even if it's no longer selling the best. You can't expect to always be number one. Others can shine while still being in decent shape after all these years.