They don't ever have to leave. This is especially true given that they left last time as a good will gesture towards the peace process and this is what they got in return.
We never do agree on this, but they left the last time because the intifada had come to a close with the factions, hezbollah in the north, actually making rare progress and general tiredness of policing Gaza. There was no peace process in 2005-06 as the US was in Iraq and Palestinian statehood was the last thing on their mind which gave Israel impunity over the next ten or so years. Build a wall and then we don't have to risk policing them (being the internal Israeli thought-process).
Try policing 2 million impoverished people who have grown up in a prison environment surrounded by freedom fighters/(slash) terrorists who despise you because you are the ones who keep them there. That's a near impossible task. Israel will become as North Korea, in the wider world, if it has to stay in Gaza for a protracted period because you already know how heavy the casualities, on a daily basis, are going to be.
when in fact this was literally Israel's 9/11 (or Pearl Harbor) which has totally changed the calculus of the Israeli-Palestinian issue going forward.
Future of the region tells me it won't matter if it was Israel's 9/11 when it all becomes much worse in the weeks/months to come.