In principle, copyright law doesn’t stop there being a system that lets you redeem the same key from Steam, Epic and Gog as long as it’s the same person behind all three acounts. There’s already a degree of precedent for this - when a publisher generates Steam keys to sell at other retailers (whether they’re codes-in-a-box at a physical shop or an online retailer like Humble Bundle), they don’t have to pay Valve a fee, but the keys can be redeemed on Steam and work just like if you’d bought the game from the Steam store where Valve would take a 30% cut. Valve probably don’t think it’s in their interest to make libraries transferrable/sharable between Steam and not-Steam, but if they change their mind, and the competitors that they’re building the transfer/sharing system with also thought it was in their interests (which is unlikely to happen at the same time), there’s nothing stopping them building it.
In principle, copyright law doesn’t stop there being a system that lets you redeem the same key from Steam, Epic and Gog as long as it’s the same person behind all three acounts. There’s already a degree of precedent for this - when a publisher generates Steam keys to sell at other retailers (whether they’re codes-in-a-box at a physical shop or an online retailer like Humble Bundle), they don’t have to pay Valve a fee, but the keys can be redeemed on Steam and work just like if you’d bought the game from the Steam store where Valve would take a 30% cut. Valve probably don’t think it’s in their interest to make libraries transferrable/sharable between Steam and not-Steam, but if they change their mind, and the competitors that they’re building the transfer/sharing system with also thought it was in their interests (which is unlikely to happen at the same time), there’s nothing stopping them building it.