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.

Those days were gone when DRM became an acceptable thing defended by people even on this site.

Its always that one CEO, wanting to emulate the success that is steam by becoming basically becoming the market and the platform. Self-delusion and believe-shapes-reality needs some realistic observation or else one wastes ridiculous amounts of money reinventing the wheel only for the crowd to flock to the first wheel shop anyway.

Those are resources that could go into inventing the first plane. And some superior sooner or later realizes that you are on a unimaginative crusade and cuts the lifeline, resulting in a dead market-plattform, with contract-bound studios being dragged down with the sinking ship.