I'm thinking more about this during the 1967 or 1973 wars, when the Jews had been living in Israel for decades. Would they have been able to remain in Palestine, sent back to their countries of origin or massacred?
I will try to answer your question without being to speculative which is always kind of hard with this kind of ‘what if’ - Questions. So let’s Stick to the facts we know:
In a speech on the 26th of May Egypt’s President Gamal Abdel Nassar told Trade Union Members that his goal in a possible war with Israel would be the total destruction of the Jewish State (Walter Laqueur, The Road to War 1967, London 1968, P. 292.). A few month earlier Radio Damascus proclaimed that Israel should be attacked until it would be destroyed (Michael B. Oren, Six Days Of War, New York 2003, P. 42). We are able to find quotes like this from all Arab Leaders from 1967.
For Israelis it was pretty clear what would have happened if the Arab Nations would have been victorious. Jitzchak Rabin (Chief of General Staff of the Army) calculated with so many casualties if the Arab Army’s would be able to advance into Israel that he ordered schools and public buildings to be prepared to function as morgues. In May 1967 Rabbis started blessing public Parks in Tel Aviv so there could function as Mass Graves if need be (Tom Segev, 1967. Israels zweite Geburt, München 2007, P. 344).
In June, three days before the war, the Chairmen of the PLO Ahmed Schukeiry threatened that after the end of a war with Israel only very few Jews would still be alive (Robert Stephens, Nasser, A Political Biography, London 1971, S. 480).
While we not know of concrete plans by the Arab Nations what to do with the Jews in case of a complete victory, we can look at their behavior under similar circumstances. When Jordan occupied East Jerusalem in 1948 it’s immediately expelled all Jews living there, destroyed Synagogues and Jewish graveyards. Beginning in 1948 Egypt started terrorizing and expelling their Jewish Population. In the following 20 years nearly all of Egypt’s 80’000 Jews had the flee the country. Similar things happened in Iraq. The Arab Nations made it particularly clear that there would be no place for Jews in their societies.
Conclusion: In the case of an Arab Victory in the war of 1967 there almost certainly would be no longer a Jewish State in the Middle East and no place to life peacefully as a Jew. Would there have been another Holocaust? Would the advancing Arab Army’s mass murder Jewish civilians? Some Israelis certainly feared this would be the case and the more openly antisemitic Arab Leaders gave them every reasons to think so. On the other non of the Arab Nations killed their entire Jewish Population (while they certainly discriminated and even terrorized them). But would the Jewish Population be able to flee to their home countries? Certainly some - but not all. Many Jews migrated to Israel from countries in which Jewish life was no longer possible and after all the hole idea behind a Jewish State was that Jews all over the world would be no longer at the mercy of other countries. You should also keep on mind that by 1967 there was a hole generation of Jews born in Israel with no second Citizenship.
Well, I hope i was able to give you some inside.
//Edit: The First Part of my Answer is largely based on an Essay by Matthias Küntzel published in German here: https://www.mena-watch.com/mena-analysen-beitraege/ein-geruecht-und-seine-folgen-zur-vorgeschichte-des-sechs-tage-krieges-teil-i/#_ftnref1