I found this comment about Dell very interesting. The author argues that Dell's competitive advantage was eroding even before smartphones ate into PC demand:
The shift from desktops to notebooks is bad for Dell long-term. Dell's big cost advantage comes from their "no inventory" system. They take your money today and buy all the parts necessary to build your custom desktop a day or two later. If you buy a HPQ desktop at BestBuy today, on the other hand, HPQ bought the parts eight weeks ago in Southeast Asia. The eight weeks is spent in manufacturing, shipping, sitting in inventory in the distribution center and finally on the retail shelf. Since PC components depreciate 1% per week, Dell has a built in 8% advantage. They also don't tie up capital in inventory and AR, but that's pretty minor these days.
The above scenario doesn't really work with notebooks. The notebooks are pre-assembled in Asia and shipped over. Dell now gets hit with parts depreciation while the notebook is made, shipped and sitting in the distribution center. They still have an advantage in that parts don't continue to depreciate on the retail shelf, but we're now talking 3-4 weeks instead of 8. And this is partially offset by higher shipping cost.
My take: Most high-quality companies aren't inherently high-quality. In a lot of cases, a competitive advantage is the byproduct of a specific business environment, as seems to have been the case with Dell. On the other hand, even a temporary, context-dependent competitive advantage can be quite valuable. Dell had a 10- or 15-year period during which its business model gave it a huge advantage, and those 10 or 15 years coincided with the explosive growth of PC sales.