I certainly believe that in the 60's and 70's there could have been a South African band on the same or better musical level as Led Zeplin. Why not ?
Well, there weren't any bands
anywhere on a par with Zep in their prime. Many good bands, many better musicians, but zep had a certain magic and everything came together. SA had Rabbitt, who were a pretty damn good band by any standards. Trevor Rabin moved on to playing with Yes ("Owner of a Lonely Heart, etc.) and now composes for Hollywood movies, while Duncan Faure moved on to the Bay City Rollers.
Also if you look around there were quite a few SA artists who've done well: Manfred Mann, Johnny Clegg, Bakhiti Khumalo, Ray Phiri, Miriam Makeba, Hugh Masekela, Dave Matthews, Margaret Singana, Jonathan Butler, James Phillips, Abdullah Ibrahim and Anton Fig spring immediately to mind. If you count in the producers and engineers like Robert John "Mutt" Lange and Eddie Kramer, the list grows even more.