I prefer the saying: Someone is going to kill your cash cow. It's better that you do it than your competitor

Milking your cash cow until it does is a sure-fire way to discover that you've lost the market. Apple have done very well by adding single new major feature every year with iPhone and then iterating on it.

E.g.

IPhone in 2007

Added 3G in 2008.

Added Speed in 2009

Added Retina in 2010

Added Siri in 2011

Looks like the super secret thing in 2012 is NFC with a use-case

It's lovely that the 2009 iPhone model is now selling for more than the 2011 Lumia 800 in local stores. That's how to retire a product.