I think the high profit margin on digital goods is almost entirely due to the more efficient distribution of the Internet vs a supply chain, not because steam enabled it. If anyone deserves that cut because of the lower cost of distribution it's the people that created the Internet, and thank God they were publicly funded scientists and not corporations.

Also keep in mind that the infrastructure of the Internet charges a usage fee, not a percentage of profit. If I change $5 for a game on steam vs $60, is steam really doing more work to justify a percentage fee?