Ok lets do an exercise in statistics not using hypotheticals but what is already publicly available knowledge.
Context
Steam reviews categorize who did the review by whether they purchased through the steam storefront, or registered a code for the game from elsewhere. When you look at the breakdown of steam reviews they give you the exact number.
Steam takes a 30% cut of games sold through their storefont. It takes a 0% cut of games not sold through their storefront that register on steam.
Foundational assumption - there is no statistical difference between steam purchasers and non-steam purchasers for how likely they are to review a game they own on Steam for any given game.
I will not be giving aggregate stats because I'm picking games at random that show up on the home page or that I think of and would therefore not assume the numbers are fully accurate to make a representative average based on. I do think they're going to be adequate for getting the message across, though.
Results
DragonBall Fighter Z - 68.7% steam, 31.3% other
Resident Evil 2 Remake - 52.9% steam, 47.1% other
Shadow of the Tomb Raider - 65% steam, 35% other
Bioshock Infinite - 70.2% steam, 29.8% other
Nier Automata - 65.6% steam, 34.4% other
Skyrim - 61.6% steam, 38.4% other
Kingdom Come Deliverence - 68% steam, 32% other
Pillars of Eternity 2 - 63% steam, 37% other
Battlefleet Gothic Armada 2 - 92% steam, 8% other
Batman Arkham Knight - 45% steam, 55% other
PUBG - 84.3% steam, 15.7% other
Day Z - 85% steam, 15% other
Rust - 74.3% steam, 25.7% other
Celeste - 91% steam, 9% other
Owlboy - 58.1% steam, 41.9% other
Hollow Knight - 86.7% steam, 13.3% other
Inside - 83.6% steam, 16.4% other
Rocket League - 71% steam - 29% other
Rimworld - 78.4% steam, 21.6% other
30% is not an accurate figure that represents what share of revenue Steam takes from games. Valve pays for the distribution of every copy registered, and gives all of it's features to all steam copies, but only takes the 30% cut on between 60% and 90% of copies sold, varying wildly game to game based on that game's individual circumstances. We cannot calculate the "True cut" because we do not know the differences in revenue between Retail / Kickstarter / Humble Bundle / Reputable Other Storefronts / Shonky Eastern European CD Key Website for a comparison of revenue, and because we don't know how much each copy was sold for on Steam (i.e. some might be frontloaded, others might be mostly sold during 75% off sales). But we do know that 30% is not the true number.
