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Only this morning, I signed up a new client. They kindly showed me round their offices. Guess what browser they use as standard? Yes, Netscape 4.79 on Mac OS7 and 8. They're not likely to upgrade soon either - they haven't ever been affected by a virus, and their ancient Macs are solidly built and ready to go on working for a good few years yet.
This 100% flamebait post was brought to you by the letters N, S and the number 4 ;)
People use what they want to use, and they won't upgrade if they don't want to. Most people can't be bothered to change browsers (if they did, then we'd have a 5% IE market share!). We can't force them if they're happy (and if you try to force them, then the chances are they aren't able to upgrade anyway, so you'll just lose their custom).
The solution is to write standards compliant code, since in most cases this will still function in N4 (albeit with poorer visual appearance).