Well, first, because tech writers are paid to make dire predictions - most of which won't come true and...
While HP and Dell’s market share can grow and shrink dramatically based on pricing fluctuations and the outcome of few big deals, Apple offers its customers a unique product with no direct equivalent in the market, making it harder for PC makers to poach Apple’s sales.
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Now that Apple is a PC vendor with growth exceeding the PC industry at large, it is in a position to convert Windows users to the Mac. For the first time ever, Mac users can run Windows natively if they choose, so there’s no remaining risk or barrier to switching to the Mac left.
Apple is by far at its strongest competitive position in more than a decade, right as Microsoft is at its weakest, struggling to cope with legacy problems, architectural flaws, and security issues, and hoping to sell its user base on a long overdue and expensive upgrade to the new and unproven Vista.
Read "Leopard vs Vista 3: A Risk Strategy" from Roughly Drafted for more insights.