Aegislash in general is arguably one of the weirdest evolutions in that it is not a "direct upgrade" from its pre-evolution statistically like most evolutions are. Aegislash is rather a very completely divergent Pokemon from its pre-evolutions, especially Doublade. Doublade is inherently a weird case where despite being a mid-evolution, by its stats you can tell it's a very decent and functional Pokemon in its own right, with a distinct stat spread that is min-maxed with high physical stats and low special stats, and low Speed, and it is a natural statistical upgrade from Honedge like most evolved forms are.
But when you go from Doublade to Aegislash, Aegislash is effectively a completely different Pokemon from Doublade and Honedge. In terms of its stat build, it changes entirely so it has low offensive stats and high defensive stats. Unlike Honedge and Doublade who had a standard ability in No Guard and were by all means a typical Pokemon with nothing special going for them, Aegislash also changes to become a gimmick/novelty Pokemon, as its ability changes to a unique signature one in Stance Change, which gives it a gimmick of two forms, where it transforms into an offensively oriented form where its usual high defensive, low offensive stat build switches to a high offensive, low defensive stat build, when attacking, and when defending with its new signature move, King's Shield, it changes back. Aegislash is effectively entirely different from Doublade and Honedge in that regard in that the way its stats and ability change turn it into a functionally different Pokemon who is also a unique gimmick Pokemon, with a unique gameplan tha neither of its pre-evolutions use.
While BST wise Aegislash is an upgrade from Doublade in that it gains 72 stat points upon evolving (448->520 BST) in Gens 6-7, and after the recent Aegislash nerf in SwSh a mere 52 stat point increase (448->500 BST), from a build standpoint Aegislash is completely and utterly different from its pre-evolutions. It is bar none one of the most unusual evolutions in that regard, as usually an evolution is a statistical upgrade from its pre-evolution, but Aegislash while having a higher BST than Doublade is effectively an entirely different Pokemon, the only thing they have in common is their shared typing and movepool by virtue of being in the same evolution line.
I don't know what they were thinking with it in particular, and I'm not mad about it personally since that's what makes Aegislash such a cool and interesting Pokemon to me, but Aegislash is definitely an extraordinarily unusual evolution among evolutionary lines in that regard and most certainly isn't like most evolutions in terms of how it changes from its pre-evolution.