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Vodafone Finds Brits Keep Mobile Phones for 4 Years Instead of 2
(www.ispreview.co.uk)
This is a most excellent place for technology news and articles.
If the batteries were easily replaceable, and the software didn't continually get bloated, and companies kept issuing security patches, sure.
I kept my last desktop system for 10 years. Actually I still have it and it performs sort of ok (I was running Mint the whole time). But I upgraded and the performance improvement was actually worth the considerable cost. I've gotten similar life out of my other desktops and laptops over the years.
I think at least 5 years or preferably 10 is reasonable for smart phones.