I was surprised to find this quote from George Bartholomew, Jr. and Joseph Birdsell (1953:495), explicitly mentioning the possibility that the spread of hominin populations was accomplished with introgressive hybridization of previous populations:
The replacement of the australopithecines by somewhat more advanced but related hominids may have followed the usual mammalian pattern of the gradual expansion of the more efficient form, and the slow reduction of the numbers of the less efficient. In many instances, however, population change must have resulted from gradual genetic penetration, and much of human evolution in the Pleistocene could easily have been powerfully affected by introgressive hybridization. In this regard it should be remembered that anatomical differences do not necessarily indicate genetic incompatability between groups, and that there is no evidence of reluctance to hybridize even between widely different human types. If rapid and dramatic group replacement did occur it must have been a rare event occurring in special circumstances.
The final sentiment, that “rapid and dramatic group replacement” must have been a rare event, is unfortunately worded—they clearly were thinking about introgressive hybridization as a slow event, “gradual genetic penetration”. On a geological scale, such episodes may often be indistinguishable in their speed from total replacement.
Bartholomew, G. A., & Birdsell, J. B. (1953). Ecology and the Protohominids. American Anthropologist, 55(4), 481-498.