PC sales are bound to taper off as PCs have long ago reached commodity status for the vast majority of tasks they are used for, and other devices (phones etc.) have become powerful enough to replace their functionality. As such it's natural for lower end parts of the PC market to level off at a relatively low rate as the market reaches a saturation point and refreshes of existing hardware are relatively slow and irregular. This does obviously not apply to the high end and gaming markets where performance matters, and that is where there is (still) a decent amount of growth. These markets will also reach saturation, but will have higher rates of replacement and upgrades as there's a built-in incentive to keep near the bleeding edge of performance.