I just don't see how this can be explain logically , people are just trying to make an excuse for it but ....until Kishi explains it (if he does it) you got nothing , which I doubt it will help that much anyways.
There is a logical explanation for at least susanoo. There is none for ms techs. Susanoo, might very well not be a ms techniques itself. When a user uses it, they dont bleed from the eyes. Instead it uses the energy in a persons body. It isnt something that requires eyesight either. It ACTUALLY was FIRST SHOWN without the user useing sharingan. Just with REGULAR(nearly blind) Eyes.
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From the understanding of what we have been shown, Susanoo is a technique, that a uchiha is able to use once their chakra has reached a certain potency. It is not even neccesarily a ms technique. A requirement to be able to unlock it is indeed the ms. It is not like tsukyomi or amatersu, where it has been directly related to being a eye technique.
As tobirama explained, the uchihas emotions cause them to produce a special chakra from their brains. That chakra when going to the eyes, manifests itself as the sharingan. That chakra enhances eyesight and provides special techniques, the strong it gets. It would make sense that when it manifests itself in the body, it is the susanoo. However just like a ms tech, it requires a strong potent chakra.
Dont get me wrong. Any other techniques, i would be among the first to cry out "plothole" because the other techs have been DIRECTLY shown to be ONLY eye techniques.
I use this logic to theorize on how its possible. I dont care if im right, i just try to think of ways it ISNT impossible. And then i wait for kishi to explain.