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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.
I think it is mainly battery life which drives upgrades now. Unless you really want the best camera. It's the only thing that seems to improve for the last few iterations.
Everything else is pretty much perfected at this point, but batteries and tiny cameras are hard to perfect. Still have to wait more than two years to see any meaningful improvements in either of those.