It would have been enough to no longer be in servitude in Egypt, and that would be sufficient reason to praise Hashem. But that there is justice served, such that evildoers who hurt us do not simply get away with it, is an additional boon, and worthy of thanks and praise.
One famous and somewhat obvious question: how we can say that it would have been sufficient for us had He not split the sea for us? Wouldn't the Egyptians have annihilated us?
To answer this:
1) Who says the showdown at the Yam Suf had to happen? According to the Biblical narrative, the situation was orchestrated by Hashem, by directing the Hebrews to travel and encamp at a certain spot, in order to convince Pharaoh that they were confounded and that the wilderness had closed them in. All this so that Hashem could glorify himself with the Egyptian defeat. But had He just given us their money (presumably when each person asked this of his Egyptian neighbor, rather than the spoils of the Yam Suf, which is later), without having split the sea, it would have been sufficient.
2) It would have been sufficient does not mean that we would be happy with that state of affairs. Rather, this is an enumeration of all the good things Hashem had does. And at each point, Hashem's actions on our behalf is sufficient reason to give praise. This is explicit at the end of the song.
In terms of sustaining us in the wilderness for 40 years, even though we would have preferred to have entered Eretz Yisrael immediately, it was a miraculous and group thing to sustain a people in the wilderness for so long. (And the stay in the wilderness was arguably a formative experience for the Israelite people.)
Another famous question is: what point would there be in bringing them before Har Sinai and not giving them the Torah?
1) I would answer that this was a fulfillment of the sign that Hashem has sent Moshe, as mentioned in Shemot 3:
|יב וַיֹּאמֶר, כִּי-אֶהְיֶה עִמָּךְ, וְזֶה-לְּךָ הָאוֹת, כִּי אָנֹכִי שְׁלַחְתִּיךָ: בְּהוֹצִיאֲךָ אֶת-הָעָם, מִמִּצְרַיִם, תַּעַבְדוּן אֶת-הָאֱלֹהִים, עַל הָהָר הַזֶּה.||12 And He said: 'Certainly I will be with thee; and this shall be the token unto thee, that I have sent thee: when thou hast brought forth the people out of Egypt, ye shall serve God upon this mountain.'|
Further, what Moshe had asked Pharaoh for repeatedly was to take the Israelites out to serve Hashem. And this could be what happened at this mountain, Chorev / Har Sinai.
2) Alternatively, the revelation at Har Sinai was sufficiently a positive experience, even had the Torah (/10 commandments) not been the end result.
3) Alternatively, there were the luchos, and there was separately the entirety of the Torah.
Dayeinu extends until the entering Eretz Yisrael -- Neturei Karta can say that Dayeinu with good kavvanah -- and finally, the building of the bet hamikdash.
The recitation of Arami Oved Avi in the mikdash extended until the entering the land and bringing the presents before Hashem -- וַיְבִאֵנוּ אֶל-הַמָּקוֹם הַזֶּה. And so too the optional fifth language of geulah, והבאתי, associated with the optional 5th cup. Could this be an artefact of a haggadah in which they darshened, with the Sifrei, the following pasuk?
The song continues.
What does this lengthy repetition accomplish? Well, stylistically, it makes for a nice conclusion of the song.
But there is also a difference in tone between the first part and the second. The first part worked it way slowly and deliberately forward. The focus was on each reason to be thankful, and so we come full stop at the end of each statement (that could have been) and exclaim Dayeinu! And then, before moving forward, we restate the new boon Hashem granted. Thus, we appreciate each great thing Hashem did individually.
In this second half, the progression is entirely forward. There is no pause for breath at a Dayeinu, and the contrast with the first part makes that clear to the singer. And there is no backwards movement, since their is no repetition of a favor Hashem granted. By juxtaposing all of the tovahs that Hashem did in this manner, we emphasize how much it is a tovah kefulah umchulpeles and how Hashem has been overwhelmingly good to us.