Less than four years ago, it seemed plausible that Apple's (NASDAQ: AAPL) iPad product line was on track to dominate the tablet market and upend the global PC market.Today, that idea looks laughable. iPad sales have been in free fall for years. Potential catalysts like the launch of the business-friendly iPad Pro lineup, the availability of cheaper iPads, and the aging of the iPad installed base have all failed to reinvigorate sales.With this backdrop, it's reasonable to wonder if the iPad's immense popularity was just a fad.