I agree. It's a mistake for Valve to reduce platform support as it becomes a justification for dropping other platforms for other developers as well.

I know that MacOS is a bit of a pain to support right now with how steam and third party applications are treated, but it would be a bigger issue for Apple to drop support if Valve maintained a strong presence on the platform via steam. With the way things are now, Apple might rip the bandaid off and just drop third party stores on their computers.