Aizen is completely inmortal. Even Yhwach recognized this. That's why, even with Kira's erasing power, Aizen would still reappear, because the Hogyoku, rather than heal him, "remakes" him, as when Ichimaru melted his heart, or when Mugetsu literally cutted him in half. Because of this overpowered inmortality, Aizen needs only to deliver a single, finishing blow to Kira to win. And since Kira, even with his stand is just a normal human, Kira would lose. To add insult to injury, if Aizen manages to trigger Kyoka Suigetsu on Kira then it would be even worse for him, because Kira would not even be able to land a hit on Aizen, nor wound him, much less defeat him.
People tend to underestimate Aizen, or generally any character from Bleach and/or Naruto, out of spite for those series and their fanbases and because nowadays hating "mainstream" stuff is seen as a good thing, and that's why they don't do much research on it. Aizen with the hogyoku is very overpowered to Kira. That's not necessarily a bad thing, since Kira is not only one of the best villains in Jojo, he is also one of the best characters.
