PS2 was 14% down in 2004, (and 8% up compared to that, in 2005)

Xbox barely lived four years

The PSP was a bit down at its fourth holiday, and it keeps decreasing in its fifth year.

While the DS was indeed 7% up in its fourth year, and suggests a strong fifth year based on the DSi boost, it is more like a proof of my post than anything. Even though technically it is a single console,  it was the new hardware that kept it alive for such a long time.

Also note, that while the PS2 had an amazing longetivity, and the DS seems to follow this pattern, these were the most popular consoles ever. It is natural, that their huge momentum lasted for a bit longer than usual. 

But the PS3 and the xbox 360 both have mediocre sales, that doesn't suggest an unusually long-lasting popularity. 

While it is possible that NATAL and the PurpleWand will really give powerful boosts, but it will only happen with almost literal re-launches, with full support, strong motion control-exclusive lineup, 3rd parties forced to use them, etc. 

But of course, this would have the same effect on us as a new generation. New hardware to buy, no forward compability, totally new style, obselete previous version, etc.

Good points.  I do think this gen will last a little longer than usual, though, mainly driven by the following:

1 - economic downturn - I expect this to add a year or two to the gen as it will likely foster climate of moderation rather than focus on trading up to new versions (I mean in the bigger picture, not the smaller HC who'd trade up to get an even more graphically intense Gears or Metal Gear).  Economic downturns tend to focus people on getting more out of what they have and finding more cost effective ways to extend value of investments they've made.

2 - dramatic cost of development driven by HD consoles - I can easily see another Wii in a normal cycle, but not another big HD jump.  Developers have been complaining a lot about high cost of development for HD consoles, and while the SDKs are good, clearly another power jump would simply worsen the situation

3 - given 1)  and 2) above I expect MS and Sony to instead stick with the current consoles spec, which I'd argue is easily good enough to last and 3 years, and instead expand the systems through extended experiences via Natal and the 'purple wand'.  This plays nicely to the idea of 'adding value' to existing investments.  Instead of buying a whole new console, controllers, etc. you instead buy less expensive extension tech that dramatically changes what you already have.  It also eases pressure on development costs and allows the industry to milk their investments in current tech for longer.

In short, I think Sony and MS are going to go down the route you suggest, reinventing the existing consoles through peripherals and focusing on new gameplay approaches vs hiking graphical power.  I think Wii Fit has been a real interesting peripheral this gen, showing how something that genuinely extends the experience (if properly backed by titles) can effectively change the core console offering and drive additional sales and growth via titles specific to that peripheral.

MS and Sony's challenge is to avoid what I think of as the EyeToy issue - if they let their new control schemes become low interest peripheral with little support, they'll have failed completely in revitilising their consoles mid gen.  But if they get it right, it'll be as though they've launched 360 V2 and PS3 V2 and I'd expect that, coupled with 1) and 2) above, to keep the consoles on the market, viably selling, a lot longer than usual.

Nintendo may come out with a new console, but they have better breathing room to do so.  However, looking at current sales and the success of Wii Fit I wouldn't be surprised if they make a concious effort to extend Wii lifetime also.