I'm not sure, but I think that as long as you haven't accepted a plunder mission, even if you went there and did refreshes etc, you can still do convoy. So if no plunder opportunity presents itself, just do convoy yourself. With the 2 free refreshes, that's 3 batches (including current one), which is quite likely to get you a SS. And as more people do that, there are more targets at the end for people who hold out longer. So kinda like a game of chicken.

And if you did get to plunder at least once, 50>35. sure, you might lose the plunder and if you don't get another, you can't even convoy anymore so 0 coupon for the day. But that's the risk you take for the increased reward potential.

Besides, population varies between server AND merged server greatly. No hard limit would be optimal for all servers, self-regulation within the server through whatever means is always the best solution. Don't ask for all your troubles to be solved by the game.