You're implying that Sales, Marketing, and Distribution is not a valuable service by saying 30% is not reasonable. I work in the electronics industry selling components. Suppliers regularly give us 30% margin, far more on some products, despite the upfront cost of making a new microcontroller or FPGA being far in excess of the most expensive video games ever made, with our value add being, to be frank, much less than Steam. 30% margin is about average for distribution, be it food, minerals, cars, or any other industry.

If I didn't have Steam (or equivalent service like GoG), I wouldn't buy new games. That's just reality. I would play the same games I have for decades. Instead, Steam has created a very effective recommendation engine that gives me a great selection. That's more than worth a 30% cut.