Tbh, screw Mac compatibility! Who needs that shit?
It only means more work for Valve, that doesn't benefit the huge majority of the gamers at all.
Multi-OS support most deffo benefits the majority. If leading companies start to support other OS'ses we might in the future see more followers. If more dev's follow, you might see a new future standard. With that in mind Linux could get more users in the home market. The only reason why me for example is still on windows is games. If Linux were compatible with everything as windows "is" it gives end users much more freedom and choise.