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.