I just bought a washing machine with a ten year warranty, to replace one made in the 80's for which there are still official spare parts (but that I felt was better off being replaced than repaired). You can find companies who are in it for the long haul, but it is true that most aren't.
For a failed game console, 3 years seems to be about average. It's about how long sega had the dreamcast out, about how long Tiger's Game.com is, etc. At some point, companies have to cut their losses.
In my experience it has been that way for quite a while when you look at lower price point items in developing areas of technology, due in part to the speed at which minor changes are iterated and implemented in new models of devices.
But I acknowledge it may not of been this way except recently, and my memory could be painting over the 'good ol days' so I am not any more bitter and cynical about things changing than I already am.