"his longevity surely has to be questioned" - you're having a laugh, right? He's played 91 tests, second only to Gregan for the Wallabies. Those two are pretty much the only two around the world I can think of off the top of my head who were playing in 1996 and still playing.
When you take into account the pack Larkham played behind, and the ponderous service he has got in recent years from Gregan, his performances have been outstanding. Until this year (where, sadly, he did go off target at times yet still managed to carve through against Aussie in Brisbane, almost setting up a winning try), I can think of just one game - against Australia in the wet - where he really went off the rails, although I think Whitaker was inside him that day, and the combination wasn't exactly slick.
I still think he has a part to play for Australia next year - particularly if the tight five continue to build. And who can blame him for kicking quite a bit - particularly against New Zealand this year, when the ABs' lineout has been average at best, while Vickerman, Sharpe and Elsom found dominance.