I think they'da stayed in trees. They DIDn't disappear.
Probably one population stayed in the trees, while the other adapted to the grasslands. One population remained relatively unchanged, leading to some of the modern apes, while the other adapted and changed, leading to new species.
(This is one answer to the "why are there still apes" question.)
With respect, Elsie... Have you ever seen the African savannah on any nature program? There's a tree here & there, absolutely. But that's a far cry from the jungle areas where the modern chimpanzees live.