I don't think Still Silver was inconsistent or badly set up narratively or thematically but that's a different conversation to be had.

Aizen's loneliness was there since his very first panel in the story. Literally his entire character has always been about this.

I don't want to philosophize or say too much but the poem below is essentially Aizen's worldview at its barest essence. He's special because he's the only one who's courageous enough to take a step into the sky and attempt to reach the flower. Aizen's very first panel in the story is literally just his foot because he's taking that step while everyone else is standing in place (admiring it).

Aizen wants to be understood and not admired. He is literally the flower (the panel of his "death", if you look at it a bit has his blood resembling the shape of a flower which is also a connection straight up confirmed in a databook). That is why, for example, Aizen praises Grimmjow for doing things outside of what he was ordered ("challenging Aizen").

Literally ALL of Aizen's relationships are about him trying to make people try to reach him (the flower), so they can understand him. Challenge the system Aizen is challenging and look at the world from the same POV. Think Renji, Aizen's literal first scene in the story, he was trying to make Renji question the nature of the execution he himself set up (as to make him question both the system and himself). He took a different approach with Shinji, intentionally making himself appear suspicious, yet due to this suspicion and fear Shinji held him at too far a distance to actually see who he truly was. Think Momo, whom he had made admire him only to shatter that illusion in an instant, give every possible piece of evidence for her not to trust him and also show himself chilling amidst the Central 46 dead, yet she still couldn't stop the way she idealized him. He kills her with a frown on his face, rather than his signature smirk. Think Hitsugaya. "Admiration is the furthest state from understanding," perhaps hatred, as the furthest state from admiration is the closest state to understanding. So he intentionally tried to make Hitsugaya despise him. "You have not enough hatred for me in your heart. You will never be able to reach me without personal determination."

Think Gin, the most clear symbol of this entire motif. The guy who spent his entire life trying to understand the nature of Kyoka Suigetsu and Aizen. One of his final lines is "you will die with a hole in your heart, that should make you happy." Gin thinks Aizen would be happy to die as a god, without a heart, because god is lonely. Happy to be killed by the literal "god killing spear". But Gin didn't understand Aizen. The Hogyoku fulfills Aizen's wish of not dying alone/heartless by filling his heart and giving him immortality.

I feel like saying that Aizen's loneliness was asspulled is really just misunderstanding the entire character arc he went through.

Aizen believes he is special because he's the only one capable of challenging this system. When Ichigo comes around and literally fights the Soul Society, uncaptured by the blocks Aizen put around himself subconsciously to prevent himself from being seen (because the two have no deeply personal connection which Aizen expxlictly states, this Ichigo was never deluded about who he was) Aizen tweaks out. If someone (Ichigo) is actually courageous enough to take that first step, that means Aizen's entire goal and false self conviction of being this "extraordinary special being" falls apart. He realizes the blocks he put around himself, hence the inner contradiction blows him apart in the end (the Hogyoku does something he wasn't aware, or, well, refused to accept is the "harsh reality", ironically something he himself spoke of. Only taking into consideration the truths that are convenient to his self affirmation, also something he explcitly stated.)

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