How is this night different from all other nights?
On all other nights, we may eat any vegetable; on this night we are required to eat bitter herbs.
On all other nights, we are not bidden to dip our vegetables even once; on this night we dip them twice.
On all other nights, we eat our meal in any manner; on this night we sit around the table in a ceremonial fashion.
The unleavened bread is the bread of our affliction, which our ancestors ate in the land of Egypt. We break a piece and say "All who are hungry – let them come and eat. All who are needy – let them come and celebrate the Passover with us. Now we are here; next year may we be in the Land of Israel. Now we are slaves; next year may we be free."
We were slaves of Pharaoh in Egypt, and the Lord our God brought us forth from there with a strong hand and an outstretched arm. If the Holy One, blessed be he, had not brought forth our ancestors from Egypt, the we and our children, and our children's children, would still be enslaved to Pharaoh in Egypt. Therefore, even if we are all learned and wise, all elders and fully versed in the Torah, it is our duty nonetheless to retell the story of the Exodus from Egypt. And the more one dwells on the Exodus from Egypt, the more is one to be praised.
So begins the Haggadah (my copy is from Rabbi Nathan Goldberg). So begins the celebration of freedom, and a reminder of the suffering our people have faced over the centuries. For thousands of years, Jews have broken unleavened bread to remember a hasty flight from slavery and a promise of greater freedom to come.
"This is the promise that has sustained our ancestors and us. For it was not one enemy alone who rose up against us to destroy us; in every generation there are those who rose up against us to destroy us. But the Holy One, blessed be He, saves us from their hands."
Is Passover diminished by the lack of evidence for the exodus itself? I think not. I think a celebration of freedom and thanksgiving is of value for itself. That memory sustained my ancestors through pogroms after pogroms, through various occupations of Israel before that. Slaves and abolitionists in America turned to the tale of Moses in their darkest hour, and a great nation emerged from their dreams and their Passover prayers for freedom.
"Next year in Israel" isn't a slogan for Zionism. Israel, like the Exodus, is a state of mind. It's the promise of something better to come. It won't come easily or soon, but it will come.
Update: Always proofread. Unleavened bread.