Alrighttttt. Twist my arm if you must! ;)
I might write a longer, more in-depth essay about this with screenshots from the game, excerpts from the myth, stronger arguments, etc. But you'll have to be sated with ramblings off the top of my head for now.
Quick overview of the legend.
Icarus and his father Daedalus were imprisoned by King Minos for being suspected of revealing the secrets of the labyrinth of Crete. To escape imprisonment, Daedalus fashioned two pairs of wings for himself and his son out of feathers and beeswax.
While they were escaping, Daedalus instructed Icarus not to fly too low because the sea air would dampen the feathers and not to fly too high because the sun would melt the beeswax.
Icarus ignored his father's warnings and flew higher. The people on the ground viewed Icarus as a god as he flew high overhead. And the higher Icarus flew, the more he felt like one. But he flew too high. The beeswax that held his wings together melted and Icarus fell into the sea and drowned. As he fell, he flapped his bare arms and watched as his feathers floated around him.
Parallels with Ascended Astarion
Astarion convinces himself that Ascension is the only way to keep himself safe, the only way to be truly free of his imprisonment. His imprisonment doesn't end with Cazador, it extends to his vampirism as well. He wants to be free of everything. Cazador. The shadows. The hunger. You get the point.
Once he takes the power from the ritual, he feels invincible. In essence, he views himself as a deity-like figure. He is a new, overpowered type of Vampire with none of the common Vampiric weaknesses that Spawn and True Vampires have.
The ritual imbues Astarion with an enormous amount of power, and yet he craves more. He wants to take Baldur's Gate. He wants to create thousands of Spawn, and black out the sun so they can rule the entire world.
His ambitions do not stop at merely having the power to defend himself. He will keep trying to climb higher, despite knowing that the Gur (and Jaheira and probably half of the other companions) are going to be watching his every move.
When met with the Gur immediately after completing the ritual, Astarion says "They'll never stop hunting me." Yet, it doesn't shake his outlandish ambitions. Just like Daedalus' warnings didn't stop Icarus from flying too close to the sun.
Astarion may not be physically chained to the shadows, but he will be watching them forever. Which could go one of two ways: He becomes a recluse, living in fear (yet again) of being hunted and killed. Or he thinks no one is powerful enough to defeat him, and ends up doing something that gets him killed by a group of do-gooders. (I personally believe the latter will happen). Either way, Astarion will fall from his sunlit sky, flapping his bare arms and watching his feathers float above him.
I've always found Ascended Astarion's epilogue title quite interesting. I read a post about it a while ago that really dissected it quite well (I'll link it here if I can find it). Essentially, "Regent" is very telling. A regent is a person that is temporarily given reign or jurisdiction over a kingdom, when the true heir is not able to rule for whatever reason. (Think Denethor II from LOTR). The Regent's power over the kingdom ends when the true heir can rule, or a new one is appointed.
The title "Sunwalking Regent" heavily implies impermanence and borrowed power. The power from the Ascension was never intended for him. It was Cazador's deal.
Astarion will not rule for eternity. He will die at the hand of his own ambition. An ambition that he genuinely believes will shield him from every danger and free him from every struggle. And that is tragic.
