I think the DLC point is the one valid argument, although nontrivial to implement.
How do you think DLC works on DRM-free games works, like GOG? The game is just gonna check if you have the DLC installed, without any real DRM.
The main issue is, this is entirely possible right now for games to do, but it won’t be integrated with steam, and needs to be done by developers themselves. I don’t know how feasible it would be for Steam to realistically do something about it, but it’d definitely be nice if you could buy a game on steam, and later decide you want to buy DLC on another platform and install it onto your steam game.