Well, not being allowed to return to your home after having hastily fled it in the chaos of war is effectively the same thing as being expelled. I find the question of how many were literally kicked out and how many left in fear before they would have been kicked out rather uninteresting. If the right to return would be an end to the Jewish state, that means that the existence of that state is based on a tremendous injustice which was committed in the past.
Sure, the surrounding Arab states are co-responsible for the misery of the Palestinians. If they'd assimilate the Palestinians, much could be gained, and the conflict could, maybe, be solved. Those states are pursuing an absolutely ruthless policy. But still, it was not from Jordan, not from Lebanon, not from Syria and not from Egypt that the Palestinians were expelled, but from present-day Israel.
Imagine Chinese people with a minority religion would come to Belgium, after having survived persecution and genocide attempts in Asia, kick out the Flemish people and create a Falun Gong state in Flanders/Northern Belgium. If they would send them all to Wallonia/Southern Belgium, and the French, German, and Dutch government would refuse to let them in. Would the French, German, and Dutch governments then be responsible for the fate of the Flemish? The primary responsibility would certainly be with the Falun Gong people who created their state in Europe.
Of course the Israelis of today are not responsible for having created the refugee problem, and the Palestinians of today must recognize that and recognize the fact that Israel is their home. But the point remains, as long as Israelis celebrate the creation of their state without any sensitivity for the dark side of that founding act (similar to the sensitivity that Americans nowadays have for the fate of the Native Americans), and as long as the Palestinians mourn their Nakba without sympathy for the Jews who fled genocide and persecution in Europe, there will be no lasting peace.