Nah, it doesn't just linearly double like that. If it takes 10 people to build, test, and support the launcher for Windows, it doesn't take 20 people to support Linux, since most of it is going to be the same across platforms. A 1.8% increase in sales also isn't the best prediction. On Steam, the vast majority of their players and revenue are accounted for by just a couple of the most popular games, and a lot of that is dictated by what games are allowed or successful in China. If your game isn't selling in China, your addressable market is actually much closer to being 4.5% Linux. That's not to pick on China, but China is a massive market on its own, and it's the difference between the case where you're selling microtransactions in Counter-Strike 2 or if you're selling a metroidvania.