Israel now has about 6.0 million Jews including those living east of the greenline and 1.5 million Arabs.  Gaza and the West Bank have about have about 1.4 and 1.6 million Arabs respectively. In total there are 4.2 million Arabs in the combined territory. The total number of Arab refugees including their descendants exceed 4.2 million located as follows; 400,000 Lebanon, 400,000 in Syria, 2 million in Jordan some of whom have been given citizenship and the rest in Gaza and elsewhere. Now if 2 million "refugees" return to the New Palestine, there will be over 6.5 million Arabs compared to 5.6 million Jews in the combined territories. This influx of Arabs will cause civil unrest in the New Palestine due to income and educational disparity and will severely test availability of resources particularly, water. These new residents will then attempt to infiltrate into Israel just as Africans are attempting to do so and just as Mexicans are infiltrating the USA.

Israel would have to double its manpower defending the fence with orders to shoot any infiltrators. There is no way that Israel would put itself into such a position.

To my mind, there would be great pressure on Israel, naturally or by Arab design.  The Palestinians would then start demanding a one-state solution and will make Israel's life hell once again.