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.
Pretty sure Steam is using early-Gmail accounting: the cost of the services they provide will continue to decrease, while the revenue per user (read: future, lifetime Steam purchases) will stay flat or increase.
Consequently, it's good business to onboard them, even at an immediate loss.
Frequently is a bit of an hyperbole, a lot of my games come from g2a and I never had one be revoked. Most of the time it's just people buying games when they are on discount and reselling after the discount has expired.
Bethesda games are a good example of that. You can pay 80$ for Skyrim VR or get it on g2a for 20$. This is just because the game is very frequently on sale for about that amount.
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.
