I would not be unhappy with this choice, but I doubt that Philander gets dropped for Rabada in the first test. SA are conservative, no chopping and changing until really forced to, Philander extends the batting line-up as well.
Also while Rabada's skills and bowling intelligence shown on the field so far are brilliant, he has not played a great deal of 4/5 day cricket particularly not in hot/humid conditions I would personally worry about his physical fitness holding up at this stage, at 20 he is not fully matured physically and has not had enough time to work on his core strengths or fitness fully. Don't need him going missing day 4/5 of a test. Unless forced to play him this series or we are 3-0 up
. he is most likely to make his debut on a seaming pitch at wanderers against Eng!