It used to be that a house with multiple PCs wasn't that uncommon. With phones & tablets there are now many households that can get by with zero PCs, and many more that can do everything they need with just one.
Real world user performance has stagnated, with hardware gains not translating into doing a given task faster anymore. A PC from three years ago isn't that much slower at what most users are doing than a brand new one, so there's no particular need to upgrade.
This is what a mature market looks like. The product is going to continue to sell for a long time, but it's not the hot item it used to be.