I'm top 10 in my server but mostly I'm able to beat all the other top10 except for two exceptions, one is in my group so doesn't matter, and one is in another group. Personally speaking i do convoys only and exclusively when i have no time to play because plundering takes a lot of real time, but when i plunder, regardless the fact my targets are supported or not, in the worst case i get 100 coupons, but happens very rarely. Most of the times - over 80% - i just get 150.
I repeat I'm not the strongest in my server neither i am top5.
Right now if your server is nicely populated and you manage to have an healthy gnw, so with 4+ groups that do it with close to even chance to win it, then plunder system is totally favourable to top10 people if they have the time to do it.
There are close to zero risk to get the horrible outcome of 0 coupons gained and since most of the people think that even a single victory there is more worth than getting 15/30 coupons + 300/500k coins, and mostly are right to think so, then they continue to do it and, at most, do like me and select convoys only when they are in an hurry.