Having spent some time in the Himalayas I would have to say that "deep, uninhabited, forested valleys" are pretty thin on the ground up there. Like every other spot on the globe, if there is a half-way decent patch of land anywhere there'll be people in there trying to make it work for them (and b*ggering it up in the process). It's just not true that there's huge areas of the Himalayas that no-one ever goes into. And the areas where the most persistent yeti stories are to be found, such as the Khumbu and the valleys up Gokyo way, are quite densly populated by Himalayan standards.
It's also true that many, if not most, of the younger Sherpas consider the yeti to be just a superstition promulgated by the old folk.
However, it is true that the Himalayas are home to many rare, elusive and strange creatures. I myself have seen snow leopard tracks in fresh snow above 4,500 metres. It had crossed our path no more than an hour or so earlier, yet the whole valley was as quiet and still as a tomb. I still haven't seen a snow leopard in the wild, yet I believe they exist. But I'm not so sure about the yeti, at least as it is most often portrayed.