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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.
The difference is that Google doesn't deliberately go out of their way to break interoperability. If Google blacklisted all other search engines from their search results - or deliberately modified Chrome so that it wouldn't load E.g. yahoo.com - then that would be a valid comparison.
"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"
That's the problem - Apple intentionally design the iPod to make it as difficult as possible for any software other than iTunes to work with it. They ship firmware updates with code specifically designed for no purpose other than to stop third party apps working with iPods. (Why do you think it's so much pain to get a recent gen iPod to sync with Linux?)