I don't see why every intermediate stage would fail.
Homo is a tad special in this regard. We have gotten smarter as we evolve allowing us to remain in the old habitat even as we enter new habitats. We, therefore, competed directly with our ancestors and off shoot populations of Homo.
Contrast this with bears. Polar bears developed the ability to enter a new environment NOT in competition with brown bears. Both bears thrive in their separate habits in which each can out compete the other. This is the case for most species.
The world breaks everyone, and afterward many are strong at the broken places. But those it cannot break, it kills. It kills the very good and the very gentle and the very brave impartially. If you are none of these, you can be sure that it will kill you too, but there will be no special hurry.
Ernest Hemingway
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