a) the captain did not train enough like they had after the Arrancar War

b) there is no Ichigo, because if not for Aizen's experiments there is no White biting Masaki, so even if Masaki and Ishin still somehow met and had children, Masaki was not hollowfied - and Ichigo isn't a perfect hybrid of all the races (he is half-shinigami half-quincy in this scenario).

So well, if not for Aizen. The Three Realms got conquest by Yhwach and blend together - the Bleach universe has ended in a sea of primordial chaos.

Unless...

Aizen, in this scenario made another goal for himself, being as strong as he is, he'd likely conclude he is meant to play an important role in the history of Soul Society. Because if he was born this strong, there has to be a purpose to that, right? There are no coincidences in this world, no blind flickers of fate. He has this power to do something with it.

What other goal an immensely powerful being like him could have? Perhaps he could start gathering some followers around himself early on, if he decided not to hide his potential, and he'd create an alternate organization to Gotei 13? Like he gathered Espada. However, this move would make Gotei to react, so sooner or later there would be an open conflict between the two organizations. Unless his new movement was actually supporting Gotei in some way and got legitimized by Central 46. Perhaps this could be some alternative Research Center because, as you said, Aizen has a thing for gathering all types of knowledge like he's chat gpt personification.

More disturbing option would be if the group around him was something like a new religious cult. Because if a single soul could be stronger than the entire Gotei13, then isn't this soul meant to rule? But this curse of action would for sure engage the nobles, and Central 46. So this little scenario would end up with violence.

Another option I can see is, that even if Aizen didn't want to initially join the Gotei, with his potential he'd get noticed and convinced to join. Or alternatively, perhaps he'd simply join because Gotei at least offered a contact with strong, powerful Shinigami, and his isolation could naturally made him be drawn to people like that.

So there is also an option for Aizen to be on a Shinigami side, in this case, not as a 'good guy' (his personality would still be cold, and detached) but we assume he has no agenda on his own to betray the Gotei. Or if he has his own goals, they align with being a Shinigami like what if he was an original Research and Development founder?

In this scenario, when Quincies attack, Aizen is still a captain fighting on the Gotei side. I don't think he'd defeat Yhwach but perhaps Yamamoto wouldn't have died at least?

Well, anyway, without Ichigo being part Hollow and helping Gotei, I think the Three Realms still would collapse in the end.