There are certainly explanations of how the answer to the wicked son is not dismissive, such as the explanation from the Previous Lubavitcher Rebbe that the message is that he wouldn't have been redeemed from there, but in the final redemption all will be included even him. And the Arizal says that each cup in the Seder corresponds to each of the sons, in order, so the cup which the whole Maggid is said corresponds to the wicked son, thus really the whole haggadah is primarily addressed to him, and similar explanations.
However, it has always bothered me how you really see that in the answer. That is the hint, but not what is on the page. If that was the main message, עיקר חסר מין הספר - the main point is missing and only alluded to.
Rather, I think the plain understanding is in keeping with what Tanya says in Chapter 29:
He should also thunder against it (the sitra achra - the source of evil) with a strong and raging voice in order to humble it, as our Sages state, “A person should always rouse the good impulse against the evil impulse, as it is written, ‘Rage, and sin not.’”
This means that one should rage — in his mind — against the animal soul, which is his evil impulse, with a voice of stormy indignation, saying to it: “Indeed, you are truly evil and wicked, abominable, loathsome and disgraceful,” and so forth, using all the epithets by which our Sages have called it.
The reason that humbling the spirit of the sitra achra is effective in crushing it is that in truth there is no substance whatever in the sitra achra. That is why it is compared to darkness, which has no substance whatsoever, and is automatically banished by the presence of light.
Indeed, we find this explicitly stated in the Torah in connection with the Spies sent by Moses to scout out the Holy Land. At the outset they declared: “For he (the enemy) is stronger than we,” and, interpreting the word ממנו , the Sages say:
“Read not 'than we,' but 'than He,'” meaning that they had no faith in G‑d’s ability to lead them into the Holy Land. But afterwards they reversed themselves and announced: “We will readily go up [to conquer the Land].”
Whence did their faith in G‑d’s ability return to them? Our teacher Moses, peace unto him, had not shown them in the interim any sign or miracle concerning this, which would restore their faith. He had merely told them that G‑d was angry with them and had sworn not to allow them to enter the Land.
What value did this Divine anger and oath have to them, if in any case they did not believe in G‑d’s ability to subdue the thirty-one kings who reigned in the Land at that time, for which reason they had had no desire whatever to enter the Land?
Surely, then, the explanation is as follows: Israelites themselves are “believers, [being] the descendants of believers.” Even while they stated, “The enemy is stronger than He,” their divine soul still believed in G‑d. They professed a lack of faith in His ability only because the sitra achra clothed in their body in the person of their animal soul had risen against the light of the holiness of the divine soul, with its characteristic impudent arrogance and haughtiness, without sense or reason.
Therefore as soon as G‑d became angry with them, and thundered angrily: “How long shall I bear with this evil congregation…,Your carcasses shall fall in this wilderness…I, G‑d, have spoken: I will surely do it unto all this evil congregation...,” — their heart was humbled and broken within them when they heard these stern words, as it is written, “And the people mourned greatly.” Consequently, the sitra achra toppled from its dominion, from its haughtiness and arrogance.
But the Israelites themselves i.e., as far as their divine soul was concerned had believed in G‑d all along.
We confront him in order to turn him around, like the spies were confronted and regretted their behavior.