A lot of commentators on the Israel-Palestine issue, including some I respect, in recent months and years have liked to make something along the lines of the following argument:
Given current demographic trends, and taking into account both Israeli Arabs and Palestinians in the West Bank and Gaza, in the not too distant future, the combined Arab population in Israel and the Palestinian Territories will be larger than the Israeli Jewish population. If a two state solution does not happen before then, Israel will find itself ruling over more Arabs than Jews. At that point it will no longer be able to be both a Jewish state and a democracy. Instead it will become an apartheid state and according to some proponents of the argument, such as John Mearsheimer, it will be ostracised by the international community and the American Jewish community as Apartheid South Africa 2.0. As a result it will eventually cease to exist as we currently know it and will be replaced by some sort of bi-national state or lead to a catastrophic war.
This argument, despite its popularity amongst intelligent and otherwise sound analysts, is seriously misguided. Why? Because democracy is about a lot more than ethnic majoritarianism. Ruling over occupied territory is not somehow democratic because when your country and the occupied territory are taken together, the population of the ruling ethnicity is slightly larger than the population of the dominant ethnicity in the occupied territory. That would be a ridiculous view of democracy.
Israel's rule over the Palestinian Territories is just as undemocratic now as it will be in the future when Palestinian and Israeli Arabs start outnumbering Israeli Jews. The idea that it is somehow less illegitimate and more democratic as long as there is at least one more Jew than Arab in the combined territory of mandate Palestine is absurd and offensive. Certainly it will be an important symbolic change, with the numbers further emphasizing the undemocratic nature of the situation, but the situation itself will not have fundamentally changed; Israel will continue to be undemocratically ruling over millions of Palestinians, and Israeli Jews will continue to have vastly more privileges in both Israel and the Occupied Territories than Arabs, especially Palestinian Arabs. If that situation will be able to be described as undemocratic and apartheid in the future when Arabs in Palestine outnumber Jews, it is just as much undemocratic and apartheid now.
As for the idea that the right-thinking states of the world and Israel's current supporters will suddenly turn against it once the demographic balance changes in favour of Palestinians, I don't think so. The international consensus view does not regard or treat the territory of Palestine as a single entity or country, even though for many intents and purposes it essentially is. They treat it as two entities: Israel on one side, and the Occupied Palestinian Territories on the other (or three, given the West Bank-Gaza split, but that's largely irrelevant to this discussion). When the population balance across pre-mandate Palestine shifts, they will not start treating Israeli Jews as a minority in a single entity called Palestine, undemocratically ruling over the majority Arab Palestinians. They will continue to see a more-or-less democratic (with the pre-1967 borders) Israeli state, undemocratically occupying the Palestinian Territories as it has done it has done for more than 40 years. Nothing will have essentially changed for them, and if they have been able to tolerate the situations as it currently stands for more than four decades they will probably continue to do so regardless of symbolic demographic changes.