The thing is, I don't think it's fundamentally an issue of morality. I think it's an issue of them feeling that by going the ascencion route you're giving him his bad ending and thus harming him, and they're decrying you for that. But because they lack the self-awareness or possibly the vocabulary to put it in those terms, or because they want to make an argument that gives them moral high ground, a lot of them put their argument in terms of broader morality. They're still wrong to attack other players though.
In what ways is Astarion harmed in the path of Ascension? Let's review:
1. Hunger. Astarion suffers from vampire hunger, of course, while traveling with the player he gets much better - he can drink the blood of living creatures in battle, Tav can feed Astarion his blood (although some "moral" persons do not allow him to drink the blood of living creatures, giving the answer to the question: "Who is really Cazador 2.0"). But, even with the ability to drink the blood of enemies, these torments don't completely go away. Only after the ritual is Astarion freed from this feeling: "I... I can't feel it. That ache in my stomach, that hunger - it's gone".
2. Sun. Astarion is very fond of the sun. Recall how eagerly he welcomes the dawn in the scene after his first night with it. You can call me as evil and immoral a person as you want (I personally don't take offense to this, it's important to me that Larian doesn't ruin the game, the assessment of outsiders has never bothered me), but honestly, I'll step on the throat of any "good" with my boot, spit on my own notions of "pride" if I have to, but I won't let it be taken away from him. The important thing is that in the "good" ending, it is this sun that burns him, practically kills him, and the mocking remarks of the companions mock that very thing. It looks like a symbol of rejection, of how the "good world of good people" treats the "wrong" person, the "freak" who, as you have previously pointed out, can only be "kept on a leash". In the Ascended finale, everything is fine - Astarion is happy, in high spirits, he feels completely free (since the larva is also now gone). What discomfort from the evil path? I was happy as a baby on that evil path!
3. Strength vs. Weakness. Astarion gets a power and develops it pretty quickly. At the party, he already knows how to turn into an adorable bat! Here, I don't even think it's worth spelling out how much better it is to be a powerful Ascended Vampire who can defend himself than a vulnerable brat with many limitations.
4. Love. Ascended Astarion has a man who is willing to give him everything. Astarion is in great need of love, and he becomes strongly attached to the only person he has, Tav. Because of his inner pain due to his trauma, Astarion cannot feel at ease in normal relationships, they are too unstable for him. He doesn't like himself, unfortunately, and when he gets confirmation of the connection, when Tav becomes his spawn and is definitely not going anywhere anymore, Astarion feels much calmer. I think if instead there had been some sort of "eternal bonding" ritual for the two where, "he who chooses to betray will die", without some sort of master control, Astarion would have agreed to that too. The ritual Astarion rejected expects Tav to abandon him, he doesn't trust them completely, he says so in the epilogue, and I think that if Tav stays with him (and Tav acts pretty condescending towards him in this scene), he still won't trust.
5. Emotions. Ascended Astarion finally stops the theatrics and acts the way he wants to act. UA adjusts to those around him, says what they want to hear from him. Even the tone of his voice is different. Astarion's laughter - the way the Ascended Astarion laughs! And, I don't know, maybe I'm wrong, but I don't think UA laughs at all. Look at his face, at his tears after refusing the ritual, or better yet, watch frame by frame, stopping the video at every frame, and then do the same with the Ascension video. Where does he feel good and where does he feel bad?
Honestly, if someone can consistently argue to me why I'm doing Astarion a disservice by helping him ascend and prove it to me, I'll eat an aspen stake. About morality - okay, I'm evil and immoral, so what's next? Does the victim's facial expressions have anything to do with my immorality? Lack of roleplay and plot rails how does relate to my immorality? Even if one proves to me that I'm a bad person, one does not prove to me that this story is well written.
You're talking to the wrong person if you want the fullest answer to that. To me I think the main argument would be that going down this path means he's always going to be in a cycle of never being satisfied with the power he has and always seeking out more power for the sake of never being vulnerable to anything (because there's always a bigger fish, a group of high level adventurers, a devil or demon or angel, etc that could threaten him). But I recognize that argument isn't the strongest. As I've said, I honestly don't like Astarion and I've never even completed this game, so talk to a reasonable Spawn Astarion fan who is opposed to the Ascension route but isn't going to be mean and ill-behaved towards you (they definitely exist, I have no doubt of this) and get their opinion if you're interested. Because again, I have no doubt that they love Astarion just as much as you have, and can write just as much about him as you have.
Here, I don't even think it's worth spelling out how much better it is to be a powerful Ascended Vampire who can defend himself than a vulnerable brat with many limitations.
