I caught that on the SA travel blogs and posted it earlier this a.m. in the Africa forum.
I love this option (it the only USA-CPT flight right now)
Far shorter than IAD-[West Africa]-JNB-CPT on SAA.
I'm (usually) much closer to DCA than IAD.
Flying DCA-EWR is easier - shorter lines at DCA, I can use Pre-Check at DCA (SAA didn't do pre-check at IAD).
With EWR as a UA hub, I suppose it's easier to "find" backup crew/equipment should there be a problem with the EWR-CPT metal (I've suffered too long with the SAA plane having some sort of issue/delay/etc.)
And, of course, right into CPT at a more reasonable hour in the afternoon/early evening than the JNB-CPT connection flight (which often put us in CPT in the later evening).
The only real issues are
(a) EWR is a hole in my opinion. And, even though I have "ny" in my username, I find the ppl at EWR surly and generally unhappy in their lives/work (I observe this in both pax and airport staff).
(b) I get the impression that the crew on EWR-CPT are senior staff and are kind of just cruising along, going through the motions. I always found the SAA ground and in flight staff to be (generally) more friendly and more helpful/attentive.
(c) Seating in premium economy (or is it Economy Plus) on UA is not generally the layout I prefer. But certainly doable even for the long flight.
All of this is "first world problem" I know.
And, you're right, unlikely to ever see SAA flying into New York anytime soon other than a govt plane flying apparatchiks to the UN. More likely to see SAA into IAD for those same apparatchiks coming from and going to DC from/to Africa and spending their various govt's money on flying them and their dependents business class or better.