It's simply like the phone market
IMO this is just a plateau effect. Regardless of OS, current (eg last five years) technology is now "good enough" for most use cases. Not many Business apps need a lot of local processing capability, and for home users there is not much need for the average punter to upgrade.
Mobile phone sales are suffering the same problem for the same reasons.
So, both the business and home upgrade cycles are longer than they used to be => lower sales.