There is no lock-in. It is normal to have accounts on multiple storefronts, and have multiple storefronts installed on your gaming PC; one can access multiple digital libraries on the same PC!

Steam wins because it provides a superior product for the end-user, not because of lock-in. Games purchased through Steam can be vetted with user reviews, supported with user-created guides and steam input configurations, streamed across devices, shared with family members, and even modded; all within the Steam experience.