Market share does not justify this valuation. Is Al Gore still on the Board at Apple?
They dominate the smartphone market. And many people don't realize "iPod" is actually a brand name and not a synonym for "MP3 player".
Market share is only part of the puzzle. Margin matters, too, and that's where Apple really shines.
Microsoft and hardware vendors such as Dell/HP/etc., may comprise a much large market share, but they operate on a much lower margin as well.