From a consumer's perspective, I'd have absolutely no qualms with South Africa leaving the competition altogether. Every game would be convenient to catch live, player recognition on both sides of the ditch would increase and I think that a move to 12 sides would be strongly considered - whether in the form of two National Academy sides or two Pacific Island sides/a PI side and a Japanese based team.
South Africa is an outlier in the competition, and their position makes expansion into the Pacific Islands, at this stage, largely unfeasible. An RSA-Arg competition and an Oceania competition would probably be most beneficial for all the Unions.
Moving back to a smaller comp would be far better for the international game too. Rugby's great strength over League and AFL is internationalism, and the current setup of Super Rugby - Internationals - Super Rugby - TRC is ridiculous. As much as I love watching the Reds and Super Rugby in general, rugby's main money-spinner in Australia is Test Matches.
But I think it'll end up being that South Africa is allowed the 6th team. The main driver in modern sports is money. College sports in the US is the ultimate example - teams play in conferences that make no geographical sense just to get a slice of a larger pie, which is why teams like West Virginia play in a Texas-based conference, despite having to transport an 120-man football contingent 1600 km's every other week. A conference that originally started in the Carolina's now stretches all the way up to the Canadian border, as far south as Florida and as far East as Indiana. South Africa has the population and rugby popularity to provide a large chunk of the money in the TV contract, which makes the whole situation viable for the rest.
While the ARU is currently rolling in cash courtesy of the Lions Tour, that won't last. Whatever decision is made will be made because of money, which is why Australia and New Zealand will cave into South African pressure. It's not geographical sense that will prevail, but monetary desire.