"I suppose that the region was more hospitable a couple of thousand years ago." They were there 4,000 years ago. 3,000 year old fabrics from the mummies there are just like those at the Celtic sites at Hallstat. There is some speculation that they were the speakers of Tocharian A/B.
That's not the way I read it. They migrated
to Cherchen 4000 years ago, but nobody is sure when they left, except that they may have been in Afghanistan in 200 AD.
Anyway, thanks for the reference