Texture resolution doesn’t map to screen/render resolution like that. Depending on the object, its mesh, the physical size (dimensions) of the in-game object, and how close your player view/camera is to it, you can absolutely see a clear difference between 2k and 4k textures for the object, even when the game is rendering at 720p or lower.

The 30% cut was industry standard for digital distribution for years. Google, Apple, and numerous other players all took 30% as standard.

That being said, Steam hasn’t taken a flat 30% for years now - their standard agreement starts at 30%, decreases to 25% after the first $10m in sales, then decreases further to 20% after $50m.

Furthermore, Valve has done more in terms of providing services, APIs/libraries, and end-user features (all with no additional fee to the developers or consumers) than any other game storefront has. I’d say they more than justify their cut.