I can get the transfers between friends part, but why between platforms? That makes zero sense from a business standpoint.
The only way that would work is to have game companies manufacture and distribute an external storage medium themselves, because platforms sure as hell won’t say “Oh you bought a license on another store? Sure, you can use our CDN for free!”. And now we’ve almost reinvented game CDs.
Don’t forget about moderation. It’s all fun and games until someone starts posting hate speech, copyrighted material, porn (legal or otherwise) or worse…