That's a tale of saturation. Toaster sales are probably relatively flat as well. Not because toaster use has declined but because people already have a toaster that meets their toasting needs. PC improvements have slowed to a crawl over the past decade in terms of real-world performance gains for the average consumer. Their 5 year old rig still manages to surf the web, stream videos, play games and send emails just fine so they see no reason to upgrade. Another 4 cores or the jump from 14nm+ to 14nm++++ isn't justifiable for the average user. That said I bet the combination of COVID + AMD really coming out swinging the past 2 years might make for a bit of an anomalous spike.
The original proposition was that desktop sales would rise with mobile sales (with a lag time), and that's clearly not the case to this point, and there are no indicators that will ever be true.
Have to agree with you there, smartphones are still seeing 2-3 year turn over that PCs no longer enjoy. Because they are small and visible out and about you have a second motivator for replacement besides straight improvement pressure. People using them as a status symbol/indicator feel the need to have the latest and greatest.
Phones, even the premium tier are a much less costly investment than a desktop PC. In addition to the $1500-$2500 for a decent desktop computer you also have all the supporting bits, a desk to put it on, a chair, maybe a printer, + the floorspace to house it all.
With a phone it's just the phone, and maybe a case. So while a household might be satisfied with a single desktop computer, every member of the household is going to want their own phone. More devices per household + higher turn over frequency = higher volume annual sales even at saturation.
