But even if that were not the case 20 years is plenty of time to cleanse Gaza, and the West Bank with it, of every last Palestinian. I'd say at the current rate it would take hardly a couple years.
Edit: to be fair, I don't know how to compare SA and Israel. Maybe you have a point, but I don't think it's that simple to impose any kind of, essentially, sanctions to Israel as long as Uncle Sam's got its back. That special relation is a pretty big trump card there. And, btw, we're still at a Democratic president. Can you imagine the Republicans letting Israel suffer arms bans and trade embargoes?