They consider two outcomes desirable:
1. All people suddenly drop dead and rot.
2. All people suddenly become infertile.
The second outcome is much preferable to them because that will allow humans to clean up the traces of their existance. Otehrwise the ecosystem will have to work on them for extra 15,000 years.
Of course their considerations are purely hypothetical since they only stand for voluntary extinction but if history is any indication, those guys have already split into two rival groups each working on a virus...
miko