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Both have tremendous market share, and are better than the competition. However, there's nothing stopping anyone from coming up with a better music store or search engine and stealing customers away from the current market leaders.
If someone came up with an online music store, sold DRM free music with a library comparable to iTunes, make it really integrated and work with any music player including iPods, back it up with a snazzy marketing campaign, can anybody honestly say that it will not succeed?
Palm should do just that if they were serious.