Earlier quoted context omitted.
Steam (and other stores, to a lesser extent) already does this voluntarily. There's an ecosystem of key sellers of various levels of sketchiness - greenmangaming, g2a, cdkoffers - check isthereanydeal.com. It's almost always cheaper than steam, and you get a steam key. Steam gets most of the upkeep cost, and none (?) of the revenue.
I'd hardly call 3rd party grey market key resellers like g2a something that steam accepts voluntarily. These keys are often stolen or pirated. The Steam subreddit has a page dedicated to it https://www.reddit.com/r/Steam/wiki/dangersofkeyresellers The keys from these sites are frequently revoked after they're discovered stolen.
However, all of the sites listed at isthereanydeal.com I have tried are not that. It's not clear what the business model is, but they're absolutely depending on steam to deliver and maintain the content, and it's not clear that steam gets any money out of the transaction.