The answer is really quite simple. Within its context in the Haggadah, the refrain of "Dayyenu" has an implicit suffix - it does not mean 'it would have been enough;' rather, "Dayyenu" means 'it would have been enough to say Hallel for...' In this "piyut" which poetically summarizes the story of Yetziat Mitzrayim, we declare that each single act of God's kindness in that redemption process 'would have been enough' to obligate us to praise Him, i.e. to recite the Hallel.
In the Haggadah, we recite "Dayyenu" at the conclusion of magid, prior to mentioning "pesach, matzah and maror" (Rabban Gamliel omer...) and reciting of the Hallel. Mentioning these three mitzvot adjacent to the Hallel parallels the requirement to sing Hallel while eating the korban pesach during the time of the Temple. Thus, "Dayyenu" serves in the Haggadah as an introduction to the Hallel:
- Had Hashem only taken us out of Egypt and not punished the Egyptians, that "would have been enough" to obligate us to say Hallel.
Had he split the sea for us but not given us the 'mahn,' this alone would have been sufficient reason to praise God.
And so on.
When we recite the Hallel at the seder, we do so not only out of gratitude for Hashem's taking us out of Egypt, but also in appreciation of each stage of the redemptive process. As the "Dayyenu" emphasizes, we thank God not only for the exodus, but also for the 'mahn,' for shabbat, for coming close to Har Sinai, for the Torah, for the Land of Israel..., and finally for the building of the Bet HaMikdash.
Based on this understanding, the "Dayyenu" contains an underlying, profound hashkafah, a message very applicable to our own generation. Today, there are those who focus only on the first stanza of "Dayyenu," viewing freedom as the final goal, the ultimate redemption. For them, the first stanza of "Dayyenu" - the exodus - is "enough." Others focus only upon the last stanza, that without the realization of the idyllic goal of building the Mikdash, the entire redemptive process is meaningless. In their eyes, Hallel should be sung only when the redemption reaches its ultimate goal. "Dayyenu" disagrees - each stage of the process requires Hallel.
It is this hashkafic message, i.e., the understanding and appreciation of each step of the redemptive process, that "Dayyenu" teaches us. Ge'ulat Yisra'el - the redemption of Israel - is a process that is comprised of many stages. Every significant step in this process, even without the full attainment of the ultimate goal, requires our gratitude and praise to Hashem. In each stage of redemption, Am Yisra'el is required to recognize that stage and thank Hashem accordingly, while at the same time recognizing that many more stages remain yet unfulfilled. "Dayyenu" challenges us to find the proper balance.
[P.S. Save this shiur! You can 're-use' it for Yom Ha'Atzma'ut.]