You can only plunder convoyers
However, support can be anyone, which means a plunderer can support the one you try to plunder and you will end up fighting that team as well.
Also, there is the issue of supply. As more people think the way you do, there might eventually end up in a state where there are more plunderers than convoyers, so each of you, on average, won't even get one plunder. This isn't an issue in the oldest servers, but for newer ones, it can be a significant problem.
And at last but not least, if there are way too many plunderers such that SS convoy almost always fails, eventually the convoyer might simply not take any SS to spite you.
So while it's true plunder have much higher potential yield, you need to pay attention to the server en environment to see if it would actually work for you.