The portions in bold seem contradictory, and while Homo Sapiens Sapiens may have been extant for 400,000 years, we have millions of years from previous species all of which birthed via the pelvic girdle. In a future without C-Sections or other means to alleviate childbirth, we'd have a higher mortality rate, and those people unable to bear children... would be gone from the gene pool. Alleviating an evolutionary pressure by making it invisible to selection for a while for some
, isn't the same as changing humanity radically. You asked if this was likely to produce a subset which cannot survive, but factors involved in childbirth are myriad.
As long as a significant portion of men find typically female hips and buttocks (shaped by the hips) to be attractive, that is a bigger factor than the mother or child surviving. Assuming nothing about the future, loss of the ability to perform a C-Section would require a LOT of damage to human knowledge. A copper or bronze knife, some alcohol and sinew, and something to act as a few clamps (including midwives' hands) and KNOWLEDGE are enough to perform a C-Section. You're not going to have a good time of it, and you may well kill the mother, but that wouldn't change the male desire to impregnate them, and that's more in the lines of real evolution than (what you believe) is a transient surgery.