But the human lineage doesn't need to split into branches for us to be replaced by another species -- we might instead simply evolve away from our present form sufficiently that present-day people would not be able to (a) recognize our own descendants as kindred spirits or (b) be able to breed with them (assuming an imaginary biological test in which one of us is propelled 200,000 years into the future).

Modern humans have existed for about 200,000 years. Therefore, natural selection being what it is, chances are in another 200,000 years we will no longer exist in our present form, but be replaced by a new species, one we cannot imagine.