> Apple gets to address their weaknesses and Valve gets a large target market.

I don't think Apple wants any non-Apple store addressing their weaknesses, especially a solution as competent and well-funded as Steam.

If Valve gains Apple-user mindshare on Mac, what prevents them from expanding to iPhones and iPads in the EU, and likely elsewhere if anti-monopoly laws get entrenched? IIRC, Services is the fastest growing revenue source at Apple.