The current lifespan is approximately 1500 years only. That can change because the extraterrestrials can see to this if they desire to shorten the lifespan further. The manipulation they have done was intended to be a permanent alteration occurring in the far future as a kind of insurance policy, so there can be no going back to normal, even if the annihilation plan becomes aborted for some reason or another. It is sort of akin to the idea of a doomsday device that cannot be recalled, at least by human beings. This provides the guarantee of earth’s destruction with regard to providing a habitat for life forms, but not disturbing things so grossly at the current time to interfere with the wrap-up and some follow-up further exploitation of humans by the alien Greys to serve their Hybrid Program for some time yet on the earth itself.