The upside of SA would be the Slam would be a huge and rare achievement once more.
It would remove the importance of AI matches though, no more SA, a weak Australia and a NZ team who cost a bomb to host and carefully make sure they get testing matches like Italy, Scotland and Wales with the odd match against teams that have beaten them. I think this would be a shame, I like AIs but if we play SA every year it really is just Australia, Argentina and NZ who will line up with a chance of a win. SA may still play and host tours, but their relevance outside 6n would be reduced. We could still play them but we could equally play France in the Autumn.
In terms of replacing them, Italy last beat a tier one nation about 5 years ago. It was SA, ironically.