I think there are different reasons for reaching each state, but they're still true.
The original cast becomes irrelevant due to popularity polls and power-scaling. It's hard to keep an entire group up to speed, especially with varying powers/abilities. Also, authors tend to start disliking certain characters. Same happened for Bleach in a sense.
The chosen one plot is common in both the west and east. It's a simple narrative that appeals to people. It's also easy to get people to rally behind the character the author wants you to - the MC.
The Moon one? That's just Asia, man.