Actually I do expect the desktop to last 10 years. I've got desktop machines that are 15 years old that are still functioning fine for the purpose they are put to. They don't need the extra speed. They chew up a bit of power compared to the more modern hardware that could do the same job.
I had a cable modem that lasted around 12 years. It only got replace this year because it started to stop holding sync. Presumably some capacitor failed in it. I upgraded to a new modem which supported a newer version of DOCSIS. That said I didn't need to upgrade as I didn't need the updated functionality.
As for smart phones becoming obsolete. Most smart phones already have more compute power than is needed. It's nice to have the newer radios and with them faster connections. But apart from some apps that are specifically designed for iOS 7, a 3gs will run everything fine. It has the compute power needed. The screen resolution is good enough for 90% of the population. This is ~5 year old product being first released in 2009. It will do what 90% of the population want to do. It's also still receiving security updates. I applied one over the air (WiFi) within the last month.
The household has iPhone 3gs, 4 and 5 so I've had a chance to evaluate all of them. We max out on the available memory when we buy them. They get replaced due to physical damage. The last 3gs got replaced because it had been dropped to many time and connector #4 would no longer stay seated. My daughter got my wife's 4 and she got a new 5. This was a repair or replace due to physical damage. The phone itself was still capable of doing everything my daughter wanted to do on it.
I reject the contention that in 5 years a smart phone purchased today will be horrendously obsolete because 5 year old phones today are not horrendously obsolete.