
They don’t want it to be competing with their new releases, even if it’s only a small theoretical competition that doesn’t actually occur in the real world, so they currently have a small incentive to kill it. This is why there needs to be regulations introduced to ensure the server software is released when they shut it down.
I think you’re right that thier commission won’t really effect the price of games at all, as that’s more driven by how much people can afford or are willing to pay for entertainment. It still could benefit gamers if the publisher/developer got more of the revenue from the games they purchased, as then the developers could more easily fund future development, especially indie games.