This sounds like it could produce nightmare scenarios for some platforms/consumers.
Say I'm a publisher; I can sell my games for 5% cheaper than all the competing platforms and let my installer/store languish. All it needs to do is let people buy the game, and my competitors can do the hard work of actually distributing the game, updating it, supporting features like community pages, friend lists, steam workshop etc.
People buy the game from me because it's cheaper, and the other platforms can pay the actual cost of supporting that license.
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.
You can already do that with Steam[0]. They allow publishers to generate keys for free. You could sell the keys on your own store and skip Steam's 30%(?) cut while reaping the benefits of their software delivery infrastructure. There's a fair use policy[1] involved, though.
You can charge a small porting fee to compensate for this.
In practice, I think this actually hurts the established guys. A big reason I prefer to buy on Steam is because the majority of my library is in Steam. A PC gaming library also has more longevity.
In general, I would be reluctant to buy multi-platform titles on the Switch. If porting was available, I would buy certain titles on the Switch or pay to move it to Switch. I would then pay to move everything over to Steam if my Switch died.
Our society is still learning about and adapting to digital goods. The end goal is obvious but unfocussed, we want digital goods to be treated like physical ones. But how do we get there? How do you craft laws to get that result and things still make sense? I'm not sure anyone knows that yet.
Say I'm a publisher; I can sell my games for 5% cheaper than all the competing platforms and let my installer/store languish. All it needs to do is let people buy the game, and my competitors can do the hard work of actually distributing the game, updating it, supporting features like community pages, friend lists, steam workshop etc.
People buy the game from me because it's cheaper, and the other platforms can pay the actual cost of supporting that license.