Well it went strong for about 5 years, and unless you're the PlayStation 2 you cant expect any console to go for much longer than that, especially when the successor is on the horizon. It didnt run out of steam early, the HD twins are simply going for much longer than a console normally would.
I don't get what people expected out of the Wii. While its peak was much much higher than the numbers it currently is bringing in, it still is selling respectable numbers, and I believe is still on pace with PS2 sales, or at least close. A higher peak = a bigger drop.