One generation can only last for so long (PS2 is an exception). Wii should have been replaced last year and PS3 and Xbox 360 this year. Waiting for Wii U has taken a toll on Wii and the same will happen to the other two consoles next year aswell.
More than that, prices are still too high which might put them in a tough place when they are replaced: not too cheap (99$) to have that mass market appeal and not that cheap when compared to a, for example, 300/349$ Wii U. And this, i believe, will help the transition for Wii U but probably slow the transition to MS and Sony`s new consoles, supposing they go after the same exact market.
If prices become more widened by the time new consoles arrive, that might help widen the market and increase sales.
Maybe a price cut for both HD consoles could have helped at least sustain the fall this year.
