It's not that the value of a membership raises exponential. It's that not taking up that offer puts you strongly into disadvantage. It becomes a must. And he is controlling it. He can destroy merchants, simply by excluding them. He improves the commodity circulation, which improves the lives of many people overall, but his gain in power influence and the circumvention of country borders and therefore tolls, will make him the most hated person. Imagine a king that doesn't comply to his demands, e.g. outlaw slavery. "Oh. That's a bummer. Then I guess it's for the best to not step onto each others toes. I'll just close my gate in your kingdom and we do both our thing. That's fine, isn't it?" People in power can't stand one thing. Other people having power over them. I wonder how long it'll take, before they anounce him to be the demon lord and enemy of everything good. My guess is before most people notice, how nice his gates are.