It being simple is besides the point. Bleach 542 Shinigami abilities can vary significantly in that regard, some are very simple and some are very complex. I mean, soifon’s bankai is a simple bazooka, renji’s most powerful ability so far is a sort of reiatsu cannon and I would think there are quite a few other abilities which are just that simple. I would think the ability is just fine for a zampakuto ability for that matter.
I mean, the manga has already mentioned concentrating power like that is not something which other shinigami can easily do. The fact that they do not do it should be indication enough of that. Kenatsu is quite common but ichigo’s ability to concentrate power on the edge of the blade and release it like he does is quite unique, even byakuya was surprised when he first saw it.
Also, you are forgetting that those capable of bankai can release their swords without calling its name. Ishin’s sword does not change shape when released, it simply releases a lot of flames. So yes, ishin did release his sword against aizen and he also certainly used a very specific zampakuto technique against him.
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