Its not Windows 10
I think this analysis is entirely wrong, the reason PC sales have slumped is because since around 2009 PCs have reached the performance targets that most people are happy with. Reliability has also improved and therefore most people live with what they’ve got. In the past one of the main incentives to buy a new computer was either to improve on the unacceptable performance or due to unreliability. I run a 2009 quad core desktop running windows 7 (upgraded from Vista) which still fulfills all my needs, reliable and fast. Why would I replace it? (Unfortunately the processor is too old to upgrade to Windows 10).
I also run an I5 laptop which does run Windows 10 and in my opinion it is the best operating system that Microsoft has ever produced.
Tablets have seen a huge sales opportunity for manufacturers’ not because they replace PCs but because they address a brand new and different market but even and these sales are starting to decline the same reason that PC sales have declined, many people now have a tablet which are fairly reliable and little incentive to replace.