You've either grossly misunderstood hominid paleontology, or you're purposely misrepresenting it.
No one has ever claimed that there will be "one" human lineage without offshoots, dead ends, and branches. Hell, even Darwin back in 1872, before *any* significant fossils to speak of were available, pointed out that the tree of life should be very "bushy", with many "dead branches" that didn't survive to present day. Here's the only figure in his On the Origin of Species book:
This represents a slice of time, with the oldest period at the bottom and new newest at the top. The branching lines in between represent life forms that branch, often die out (each "twig" that stops in the middle of the diagram represents an extinction), and a few of which survive throughout the period of the chart (by reaching the top).
Part of his discussion of this diagram uses a metaphor:
The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life.In other words, each family of creature branches off into many variant forms (species or subspecies), yet most of them die out and only the most successful survive to a future date to begin the cycle anew.
Darwin thus predicted that there would be numerous extinct humanoids, *most* of which would not be on a direct ancestral line to modern humans. This is pretty obvious when you consider how evolution works.
So why are so many folks still unclear on this 130-year-old concept?
Many humanoid fossils will be from offshoots of the human tree which succeeded for a time, but eventually perished. We're the only branch that managed to survive to modern times (although it can be argued that the Australian aborigines are an offshoot which is still hanging on from when they split off about 600,000 years ago).
The reason that new humanoid fossils are great discoveries is that they help fill in our family tree, about which little was known a 100 years ago -- *not* that they may necessarily be our precise ancestor (instead of a great-great-great-etc.-uncle), although some will be and we'll know better which is which after we discover more samples from the tree.