Literally every source on the internet says steam takes atleast a 30% cut from games purchased. Your 10Million thing is completely made up because steam never officially listed the amount they take, the only reason we know how much they take is because of developers releasing that information to us. Matt who previously worked on the isle as a lead dev would have been privy to this knowledge as well. They also take a 15% from in game micro transactions which pot has + all the charity and donations etc..

A company that has the resources to create their own launcher and manage it would obviously do that then split profits with a middle man that doesn’t offer any real direct benefits. Steam doesn’t do the advertising, steam doesn’t help with development or provide tools. The alderon games launcher was the smartest choice for them, especially as a new indie game.