An aside: The number of Jews in Palestine at Partition was well below a million. The UN Special Committee on Palestine estimated the Jewish population at 608,000 before partition.

I do like your question, though. In every course taken and book I've read on the matter, the explanation is typically the Arab armies were disorganized and had conflicted goals (I just double checked Charles Smith's Palestine and the Arab Israeli Conflict to confirm my recollection). The Jordanians fought somewhat half heartedly, and had secret contacts with Ben-Gurion's nascent government (Segev, 1949: The First Israelis). The Jordanians were happy to accommodate Israel so long as Jordan got to expand its territory and they could keep Egypt in check.

To my knowledge, no one has done research comparable to Benny Morris' research on Plan Dalet with respect to Arab plans.

Edit: I should clarify I was only writing about 1948.

I more or less answered this here. But the problem is that this is a "what-if". We can't know for sure. Some previous statements said the goal was to deport any Jew who came to Palestine after 1914, for example. So it's not certain. But it most certainly wasn't all that great. Also note that each country had different plans for what territory it wanted, so there's no guarantee what each would have done. They were united in name but not in goal, and they were poorly coordinated (one of the reasons Israel managed to fight them off, actually).

I should note that Arab archives, unlike Israeli archives, remain entirely closed. It's speculated by many historians that records on these subjects won't be found regardless, because those historians believe the Arab states did not keep the records well. Israeli archives themselves are not fully declassified, and the records get released periodically and are censored to some degree as well. But getting at Arab plans is nearly impossible. As historian Benny Morris wrote in Birth Revisited:

But a major problem [in writing this book] remained: Arab documentation. Unfortunately, the Palestinians failed to produce and preserve ‘state papers’ from 1947–1949, and the Arab states – all dictatorships of one sort or another (military juntas, absolute monarchies, etc.) – refused and continue to refuse access to their papers from the 1948 war, which they regarded and still regard as a humiliating catastrophe. In the course of the research and writing, I did my best to illuminate this ‘area of darkness’ by culling heavily from Jewish or Israeli intelligence material and British and American diplomatic dispatches dealing with the Arab world...

Having spoken to him, he doesn't believe many of the archives even still exist, but is thankful that Israeli archives have opened more over the years. So that is a major obstacle to figuring out what these plans would be, because we simply cannot access documentation that might tell us.

Highly contentious question as the conflict and historiography is obviously very politicised. The way I see it, there are essentially two schools - one (Anita Shapira, Tom Segev, Benny Morris in recent years), going off pronouncements like "we will throw the Jews into the sea" and the idea of a true me'atim mul rabim (the few against the many / David vs. Goliath) believes that in the wars up until 1982 (Lebanon being the first "war of choice") there was a very clear existential threat to the Jewish people: in 1948 and 1973 this would have meant partition between Israel's neighbours and doubtless mass murder.

The second school (Walid Khalidi and Ilan Pappé most notably) believes, especially in reference to 1948, that this was never a possibility as the Arab armies so disorganised, ill-trained and lacking central and modernised command, and the Jewish forces having built up functional proto-state institutions under the British mandate and operating under a cohesive and unifying ideology (with diaspora backing). Essentially, they shy away from the counter-factual and call it a redundant question (to see events as this inevitable is in itself overly mechanistic and almost teleological but that's another topic).

Personally I think it naive to believe there wouldn't have been a huge and arbitrary slaughter given the animosity that was already in place by 1948 and only continued to grow. Not the entire destruction of the Jewish people as this would have meant worse repercussions than any possible benefit, but certainly a significant reduction in their population and assimiliation into various Arab states as a powerless minorirty. In terms of land, this would have, as others have argued, been split between mainy Egypt, Syria and Jordan, perhaps with Iraq and Lebanon receiving a smaller share. Depends on the relative power of each in whichever war you're talking about. It is doubtful a Palestinian state would have been created in its stead if we take what happened in 1948 as a precedent (annexations by Egypt and Jordan).

Anita Shapira - Israel: A History

Ahron Bregman - Israel's Wars

Benny Morris - 1948

Ilan Pappé - The Ethnic Cleansing of Palestine

I'll avoid repeating what others have already said here but I wanted to add that there is already some indication of what would have happened because in a way it already DID happen. Before 1948, there were tens of thousands of Jews living in cities like Hebron and Eastern Jerusalem which are popularly considered to be the West Bank. During the first Israeli-Arab war, Jordan annexed nearly all of the West Bank, land intended for the Palestinian state, and took the eastern half of Jerusalem. What Jordan did after this is one for the history books: Instead of accepting a Jewish minority in the land they rounded up every single Jew and expelled them from the country. Literally every single one. Historical Jewish holdings that had been owned since the middle ages were ethnically cleansed by the Jordanian army. Today, the only Jews in the West Bank and East Jerusalem are those settlers who came back after 1967. It's worth noting that these pre-1948 landholdings are the basis for the majority (but not all) of Israeli settlement claims in the West Bank. For the most part (again, not universal) the land in contention, like Gush Etzion for example, was actually historically Jewish land that was only removed of its Jewish presence through a deliberate tactic of ethnic cleansing.

So in a way, we already have our answer - they would have expelled them.