but at the same time something kills all the bamboo (human or otherwise) and they are scuppered

They are very very well evolved for a very small niche. Humans aren't the only force which can change niches and start exerting negative selection pressure. Their very slow reproduction rate probably wouldn't help here

There is also the pragmatic view that Pandas might already be past the point of new return and we are really swiming up stream, would we be better spending more money on species which are easier to save or provide greater benefit to other species or humans