Even if you're replacing the software, having the source allows you to ensure that your replacement is fully backwards compatible with the original, and, if it is not, enables you to implement a compatibility shim that fixes those issues.
That said, OS X being exactly what Linux dreams to be (Unix with a beautiful and useable UI, scriptable apps, reusable components, and a modern development platform), I know I'm not the only one to whom it provided a haven after years of Linux-induced frustration.
You can't just blame the Linux community, though. I agree that the community could do a lot of things better (especially with regards to UI development). However, one big obstacle the Linux community has to work against is hardware support. Windows is the dominant OS - manufacturers essentially subsidize Microsoft by providing Windows drivers for their product. Apple, by choice, writes OSX to only work on a very limited subset of devices that have been approved by Apple. Linux has neither of those advantages.
In theory, the fact that Linux is open should make it easier for programmers to make their own drivers and release those drivers to the community at large. In practice, because of many of the concerns that you've cited, proprietary drivers still outclass Linux drivers for a number of components, including WiFi, ACPI power management, and graphics.