When/if I ascend him a 1 minute long cutscene in the end is certainly NOT the decision making reason. Whoever this writer is she should NOT assume that we all have the same reasons. It certainly speaks volume for her would be reasoning to ascend him, i guess.
This whole drama with "good"/"bad" ending is overblown. Astarion has NO good ending as of right now. Things are very vague in this state of the game, nothing is explained as to what is going to happen to them (Tav/Astarion).
If he stays a spawn and you're an elf than you've got around 700 year to stick together, if you're a human for example then in 40 years you're old and in 50-60 you're dead. And where does it leave him? Alone in the shadows and with a never ending hunger for blood. Well, does that sound like a happy ending for him? There has to be a HOPE in the so called "good" ending to have him cured of vampirism. A wish spell/ spell of true resurrection, or a Gale if he was ascended to a Godhood. Otherwise it's all down hill for him after Tav is gone.
And I don't even want to get in to the ascended Astarion debate until there is an OFFICIAL stand from WOTC about what the ascension actually means and does to a vampire.
As of right now there is NEVER been an ascended vampire, the ritual has NEVER been performed before. There is no lore on the ascension just a lot of speculations. All we have right now to go by are Raphael's words that the ritual will make him a Living Vampire with all the desires and arousals of men. So basically a "super-man"

. The arguing will never end until either WOTC or Larian define what the ascending means for a vampire.