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.