The PS3 can last as long as Sony wants it to.... whether it is actually worth dragging out it's life is up to the consumer though.
For instance, the PS3 is currently about 22 million, lets say it ends 2009 at 30 million, 2010 at 38 million, 2011 at 44 million, 2012 at 46 (by this time new console are out by all 3 HW manufacturers) then the next 4 years it sells an average of 1 million.
Well done Sony, you stuck to your 10 year plan, doesn't mean jack if it sold next to nothing in the later years though.
The NES was manufactured and shipped for 20 years!... but the first one or 2 were Japan only, or just Japan and America, and the last 7 or so it sold in minute numbers and only in Japan.... so those years didn't really help it's sales or it's software support.
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As for what Sony was expecting of the PS3... I think the initial production tells it all.... Sony had intended for 6 million PS3s to be shipped by March 2007, they managed to manufacture 5.5 million, but only shipped 3.7 million, and under 3 million were actually sold.
