"The chimp line could have diverged earlier (as molecular data suggests it did) and Ardi is simply an intermediate species along the line from last recent common ancestor with chimps and modern day hominin (of which the only extant species is us)lineage. Strictly speaking chimps are an "offshoot" (or reciprocally ...the hominin line is an offshoot) from the last common ancestor. Molecular data has this occurring some where between 4.5 and 6 mya so evidence of the true origin species could still be in the ground preserved some where OR it simply was never captured. Don't forget that having anything at all preserved is a geological super mega lottery, the molecular data has already told us the general story of what happened...the sparse anthropological data is just filling in the details between the milestones as a bonus at this point. ;) The controversy that anthropologists are making over it would be moot if they could get some dna from the finds. They can then definitively determine if the gene line is ancestral to ours and or chimps. Then there is the possibility that chimps could be a de-evolution of a previously advanced state in the last common ancestor some sort of tree living great ape.
As usual interpretation of the finds is muddying the waters of the discovery. Was ardi an immediate ancestor of ours ? maybe. Did modern chimps evolve from ardi's line? maybe or did they predate ardi by connection through an older common ancestor? maybe Does this discovery kill the Savannah hypothesis as the lead researchers are claiming ? nope. the simplest explanation tends to be the best..to go from an elegant ecological change leading to bipedalism to a complex interplay of food for sex makes things more complex, and more complex means more improbable, possible yes..but still more improbable. Now if a specimen is found around the 6 mya sweet spot that looks more like ardi than a chimp then it simply means chimps are a devolution of the ardi body form possibly to adapt strongly to the jungle living that chimps do..meanwhile the homonin line diverged into the savanah forms that eventually led to us. All fun stuff indeed but as usual in some what soft Sciences like anthropology, interpretation of results is what breeds the controversy!!
My guess for why we can't find fossils that represent the chimp root point is the difficulty of preserving bones in the jungle habitat that chimps inhabit...unlike hominins which lived away from trees more and more and were able to have their remains preserved in places where they were not subject to total elimination by the environment."
So the conclusion that ardi is even our ancestor is not definitively proven, it is possible that during this period there were many variant populations of hominins, of which Ardi simply was a line that diverged and then went kaput as the rift valley continued to form and the Savanah habitat emerged as a result. We know that evolution does not occur in the neat "tree" fashion that older descriptions of lineage used to portray, the actual behavior is more like an interconnected web or bush of lineages (see image),
cross breeding in many cases to form short lived intermediate forms , many of which were never to be fossilized so that we can find them millions of years later. So though Ardi does appear to be a primitive ancestor along the line that evolved Lucy it does not mean it is precisely such a species. We can make such links between us and more advanced hominins like Homo Erectus which successfully left Africa and have finds preserved in different habitats and times since the earliest dated remains are found with morphological continuity. A known example of a parallel hominin line is the Neanderthals of Europe, this robust modern species evolved from ancestral populations of Homo Erectus separately to the European climate conditions millions of years after leaving Africa, we know they are not ancestral to homo sapiens but are cousins on a side branch. It could be that Ardi is precisely on an older side branch from the line that led to us but the dearth of finds of OTHER side branches that likely existed at the time makes it more difficult for us to make any definitive ruling. By the time of Erectus there were no other side branches at least according to fossil evidence but the further back the finds go the more likely there was a higher diversity of similar populations particularly at a point of increased geologic change as was provided by the rift valley formation initiation. Unfortunately molecular data can't help provide more information on this stage in history but comparative analysis may reveal some aspect of the diversity or we can hope that more finds are made of older or comparable dated fossils of still yet other species.
So though the find is very exciting it doesn't magic bullet anything, we'd need more samples from possible hominin species that existed at the time to whittle down the relations between chimp and hominin and between the various hominin's and us.