Honestly, I just think public isn't sometimes best at forming groups. Because they'd usually just vote based on visuals, personality, story but a group needs all the positions covered, vocalists, rappers, dancers.
Say whatever you want about Izone being rigged, but they had very good composition both visually and talent wise. Meanwhile, Kep1er is dealing with lots of infighting due to Huening who objectively shouldn't have made it. And when you have a divided fandom new fans are being turned off too. So hence the declining album sales, not the songs as popular opinion suggests. If the songs prevented albums from selling Nmixx wouldn't be selling any. Kep1er's decline has nothing to do with the song reception.