Anyone who thinks Linux is going to become a mainstream desktop OS is dreaming - there is just no way. I used it for all my work for some five years, and the instant OS 10.2 came out I sold my PC and switched. Some people like to tinker with computers, and those people like PCs and Windows or Linux. I use my computer for work, and I don't have time to waste on "configuration" or "optimization".
Apple is not going to die. They produce the best professional and home machines on the market - I don't care about pure performance, because performance is a fallacy anyway. What's the point in rendering a video five minutes quicker if you spent ten minutes just getting the computer to work and rebooted twice in the process?
If Apple wanted to increase their market share, they could do it easily by compromising on quality to reach a lower price point. Say goodbye to your well designed cases and internals, say hello to ribbon leads, plastic facades and tin panels. Forget ECC RAM, forget high resolution displays (plenty of PC users read size, and don't notice the screen looks like a balloon and flickers like crazy), forget Firewire, forget Gigabit Ethernet, one fan at high RPM (can't hear it in the shop, but wait till it gets home!) can replace seven. Get rid of the door, just have screwed on panel.
All of a sudden you have a cheap, fast PC - wait, why would you buy that from Apple?
Apple is the only computer manufacturer right now selling real quality and innovation (alienware assembles, they do not invent). That's why I'm buying from them, and I will continue to do so until that changes.