[Razengan]

Many people who like Macs will continue to buy Macs even if OS X was freely available on other PCs, and OS X would continue to indirectly generate revenue for Apple even after it were open-sourced:

For one, many more people will have access to the Mac App Store and the iBook Store, leading to increased sales for apps like Final Cut and Logic. There'll be many more potential customers for their iCloud Drive storage plans. Last but not least, it will drastically lower barriers for iOS/tvOS/watchOS development as people will be able to develop on any PC they want, not to mention it would increase the pool of people making native Mac apps as well.

Apple can still differentiate Macs through their hardware, things like their pressure-sensitive trackpads, form factor and by keeping bundled apps (like Photos and stuff) closed-source and exclusive to Macs.