Based on the scan i provided Sasuke was going blind FASTER because of the use of his eyes, as Itachi mentioned that regardless, awakening the MS meant eventual blindness.Sorry, but your argumentation is incorrect. The first thing is that Susanoo is classifies as a Dōjutsu(eye technique). The second contention is that Sasuke became blind because of Susanoo. Susanoo quickened the sealing of Sasukes Mangekyo Sharingan, it overused it. That was mentioned by Obito and is a fact.
Even so, I liked your thesis.
The fact that not only Madara , but Itachi also accessed Susaanoo without having the MS actually active lends weight to the theory.
Susaanoo is classified as a Doujutsu because it is unattainable without an MS , but an active MS is no longer needed once attained.
A regular sharingan user cannot copy it etc etc
No quite, especiially when there is multiple logical explanationsno matter how people try and justify it, it's still an asspull. the special chakra is needed from the brain to activate the eyes yes, but that chakra is not what makes the techs work. the point of the dojutsu is that it acts as an conduit for said techniques. if there are no eyes there shouldn't be anything to produce said jutsu. example when using susano it put a huge strain on sasuke's eyes and thus susano vanished from the strain it put on them. i don't care so much that itachi didn't have MS activated as he still had his eyes, but madara had no eyes so thus shouldn't use dojutsu.
Sasuke mentioned he had massive strain on, and i quote, ''Every Cell In His Body'', and clutched his eyes.
Sasuke if you also remember was using Doujutsu such as Amaterasu and Genjutsu since the battle with Bee up to the Raikage and Danzo and then Kakashi
like i mentioned above the Sussanoo requires the MS to be attained by the user, but can be manifested as a jutsu of the body of the wielder
It is all theory end of the day, but carries logic to it and the fact it was done more than once, makes it more cohesive