The thing here is there is nothing wrong with plunder system. It's really fair actually. Every day you have 3 chances to plunder, no more no less, no matter if u fail or succeed in your plunder. Myself is p2w player and a plunder. I do plunder almost every day so you could call me a *. However, I doubt you would know how much coupon I got out of plunder per day. It's only about 50 to 100. Sometimes I got 150 on good day. Why? Because people on other group get smarter, they keep asking their top players helping them or pick some weird time, like waiting time between event to do convoy.
So it's fair and square here. Coming back to your argument that because of this plunder system, many people in your group have left. This argument is fallacy. It's not because of this system that people in your group left but because the players that just left are just competitive and self-center in their nature. Even if this system is still like the old convoy/plunder system, there would be a time they would leave you guys. To prove for my point, in my server, when it was first merged with 3 other servers, there are like 20 or 30 people willingly left their group to join other group because they know the other group that they will join can give them better benefit, such as more group skills level, grouping with players that are as strong as they are.
So don't be mistaken here my friend that the plunder system ruins your group. But instead, you should realize that it's just because of the competitive nature of those players that have left your group