The thing is Israel has no intention to help the people of Gaza. They have literally said so. So if they will not help the people of Gaza, they will either do exactly what you say is not an option: leave Gaza to their own fate, or they will take the land without helping the people of Gaza, which amounts to ethnic cleansing.
To be in the right (and I say this lightly, as taking Gaza would definitely be nothing more than occupying a country through an invasion), Israel needs to either claim Gaza and make the Gazan people full citizens of Israel, including all the rights that come with that (which is, if other parts of Israel and the occupied areas of the West Bank are anything to go by, is not going to happen), or they need to leave Gaza alone, including the removal of any blockade or occupation, and give Gaza a realistic chance at being it's own state.