It's not the matter of a locked platform per se. Releasing a game on a platform like Steam just increases the likelihood of someone buying it manifold. The age

It's not the matter of a locked platform per se. Releasing a game on a platform like Steam just increases the likelihood of someone buying it manifold. The age of physical discs are over, but the developer is still free to release a DRM-free version of the game independently. Witcher 3's Projeckt Red does this. You can simply enter the product key received from Steam to download this DRM free copy.