And how would a launcher identify you've actually purchased the game? You still need a central source for that. Hypothetically I guess there could be an activitypub like protocol that all storefronts could use to sync purchases, but that opens up a whole other can of worms, such as account linking, purchase duplications, refunds. The main questions with this hypothetical are

• Why would stores implement this when they don't really benefit from it?
• Why would the users want it when it means creating more accounts and linking them? Why not just stick to one platform that best covers your needs? I guess there would be the "what if Valve turns bad?" argument, but company turning bad is at best a once in a decade situation. If that's the only reason then the feature won't be used 99.99% of the time.
• There's also a question of who pays for the data? Games are huge and the cost of keeping storing them is factored into the price of the game. However, if you buy from store A and download in store B how is store B supposed to stay afloat when they only eat the cost of storing the game.

As for going completely launcherless, how do you solve updating the game? Steam was originally made to solve the patching problem, because each patch would effectively shut the entire game community down while everyone waited for everyone else to patch their game.