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The second time Aviendha went through the pillars, she witnessed the downfall of the Aiel. It is later said because the Aiel were left without a purpose at the treaty before the Last Battle. But the treaty was not something that can't be changed after they won, especially the Dragon won, and gain godlike power.
So, why did the vision show the future as if the Dragon was no longer around?
The vision didn't ignore the Dragon's role, it ignored Aviendha's
Aviendha's vision was not the definite future in store for the Aiel, rather it was a possible future, and Aviendha treated the vision as a cautionary warning. Rand's original plan for the treaty was not to include the Aiel in it at all. This led to the disastrous war in the future. From Aviendha's vison:
Alalved shook his head. “The spears grow restless. The Aiel are not meant to grow fat in lush lands, tending crops. We are warriors.”
“The Dragon asked for peace,” Tavalad said.
“The Dragon asked others for peace,” Alalved replied. “He excluded the Aiel.”
Because Aviendha had her vision, she pushed Rand to include a position for the Aiel in order to avoid the war she had foreseen.
From A Memory of Light:
"There is yet a greater issue here", a new voice said.
Perrin turned, surprised. Aviendha? She and the other Aiel had not participated in the arguments. They’d only watched. Perrin had almost forgotten they were there.
"You, too?" Rand said. "Come to walk on the shards of my dreams, Aviendha?"
"Don’t be a child, Rand al’Thor", the woman said, striding up to place her finger on the document. "You have toh".
"I left you out of it", Rand protested. "I trust you, and all of the Aiel".
"The Aiel aren’t in it?" Easar said. "Light, how did we miss that!"
"It is an insult", Aviendha said.
Perrin frowned. She smelled very serious. From any other Aiel, he’d expect that sharp scent to be followed by a pulled-up veil and a raised spear.
"Aviendha", Rand said, smiling. "The others are about to hang me for putting them in it, and you are angry for being left out?"
"I demand my boon of you", she said. "This is it. Place the Aiel in your document, your ‘Dragon’s Peace.’ We will leave you otherwise".
Why wouldn't the treaty be changed after the fact?
Who was going to change it? The Dragon was probably the only person or entity with the authority to make changes, and he died fighting the last battle. Everyone knew he would die, the prophecies had foretold it, and Rand consistently said he would die to anyone who would listen.
Although it's true that
Rand did survive by swapping bodies with Moridin. However in this state, he no longer looks like himself, and can no longer wield the Power, so no legal entities would believe he was the Dragon without a lengthy confirmation process which may have been impossible fifty years later. And he evidently had no wish to continue as a world leader, given that the books end with him riding off into the sunset.
Even if the Dragon did survive, the war Aviendha saw didn't begin until a generation after the last battle, at which point everyone else who had signed the treaty was dead, and their heirs and successors may not have recognized the Dragon's authority.
The future war started with the Aiel attacking the Seanchan. If the Aiel are the aggressors, none of them are going to want to suddenly force peace on themselves by joining the treaty. Such a decision would likely be incredibly dishonorable, like fleeing from a fight. None of the other nations would want to try to force peace on the fearsome Aiel, and any attempt to do so could lead to war with the Aiel.
Answer heavily spoilers for the ending of A Memory of Light @rand Al'thor
the dragon dies, Rand leaves the camp at Shayol gul as mere Rand Al'thor, he is not longer the dragon, he is no longer required to help the world, he no longer wants to, hes free from the burden of the pattern finally.
Aviendha's vision is based on Rand not including the Aiel in the treaty.
Could the Aiel have made their own treaty after the fact? Yes
Would the Aiel have made a treaty after the fact? No
Without the treaty Seanchan had toh towards the aiel for leashing so many wise women (while mostly shado) and a war between them was inevitable. Rand's treaty locks the Aiel out of such a war, This is why Aviendha pushes for the them to be included in the treaty to save her people.
Could Rand have intervene after the battle of shayol gul to help the Aiel? Yes
Would Rand intervene? No
