The reason Brazil, USA, China and Japan always remain on top. It is because of course they invest a lot in volleyball and they have millions of girls playing the sport. But mostly important, the reason they always stay among the top teams is simply because they have no competition in their region, so their spot in any major tournament is always a guarantee.
Netherlands finished 4th in the OG and WCH. But now they did not qualify for the OG. But if they had to play against Thailand, DR, Argentina, they would have qualified easily. And once in the Olympics they could still fight for a medal.
Brazil, China, USA and Japan all went through some generations that were not so good...but still they played in every major tournament. European Teams can`t afford that. In order to compete or stay on the top they gotta truly be the best of the best.
Take Serbia as a example. It is uncertain if they will be in Paris. But if they had to compete against Argentina, DR, Thailand.....It would be certain they will be in Paris because even if their level drop, that would still be enough to beat Argentina, DR, Thailand,Korea.