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.

In my situation, which I described, right post-merge, I was about 9th in rankings, I believe, with 40-50k power range.
Our top 3 are 100k, 90k, and 84k (estimates). Those 3 people are in other group than me.
I do plunder, and all 3 tried are protected by them. That was my plunder record for first 2 weeks of the merged server. Now, when my server is just much "slower", although not "dead" per say yet, it just takes too much time to do plunder.

Where I'm going with it, it went from "fast but impossible = 0 coupons" to "slow, but maybe 50 coupons if I luck out and find someone".

The situation you describe could technically work, but it depends on where the rest of the top 10 is in the groups. I understand how it works for you, the situation I present is just my perspective of the system.