"The PC market as a whole is shrinking, but desktop sales have halved in 10 years."
There was a time when PC hardware capabilities changed quickly - and each gave a significant performance improvement. My current desktop was a fast PC 10 years ago. For my non-tablet needs - performance would not be significantly improved by using a newer desktop offering.
My stock cupboard has a couple of reserve motherboards/graphics - returned from users whose mundane needs are now fulfilled by smart phones or tablets. Only the few dedicated gamers want desktop upgrades to support video cards with a faster bus.