The market leader always want to extend the generation because the last few years is were all the money is being made. If the PS3 continues to do poorly sony will be first out the door with the PS4 to try and start over and win the next gen. The way things are going right now MS will want to keep the 360 around as long as possible to collect end of generation money. MS had no reason to keep the xbox 1 alive. It was a money pit with no end in site they were getting their tales kicked by sony. You think sony would be any different? You think they will just sit there with a failing system and eat cash for no reason? Market share and profit is going to determine the life span of a system.
The biggest reason MS did not make the hard drive standard was so one day they could get the 360 to 129 or less and still make a profit like sony is right now with the PS2.
Another positive aspect is that you can use to the profit generated by the older hardware to subidize and reduce to hit of the initial cost of the new hardware.
Imagine how bad Sony's numbera would look with no PS2. Imagine how MS's numbers would look if the xbox had been as viable as the PS2.