Dr. Pilbeam may have been misquoted don't know for sure but this is the quote that your side is giving.
"Of the primates, the chimpanzee is man's closest relative, while the two other great apes, the gorilla and orang-utan, are slightly more distant evolutionary cousins. The apes and hominids are collectively known as the 'hominoids'. Biologists would dearly like to know how modern apes, modern humans and the various ancestral hominids have evolved from a common ancestor. Unfortunately, the fossil record is somewhat incomplete as far as the hominids are concerned, and it is all but blank for the apes. The best we can hope for is that more fossils will be found over the next few years which will fill the present gaps in the evidence. The major gap, often referred to as 'the fossil void', is between eight and four million years ago.
David Pilbeam comments wryly, 'If you brought in a smart scientist from another discipline and showed him the meagre evidence we've got he'd surely say, "forget it; there isn't enough to go on".' Neither David nor others involved in the search for mankind can take this advice, of course, but we remain fully aware of the dangers of drawing conclusions from the evidence that is so incomplete.
Fortunately, there is quite good evidence regarding the ape-like creatures that lived over fourteen million years ago."
EvC Forum: Quote mining? The Pilbeam quote...
If they were better adapted why did they go extinct ?
So your theory rests on maybe.