"Almost all" of a specific group amongst the Uchiha, not "almost all" Uchiha. And he didn't have knowledge of Black Zetsu, however, Black Zetsu's existence doesn't exclude the existence of the Curse. The emergence of the Sharingan itself in the individual has zero to do with Zetsu, the chakra itself isn't related to Zetsu. And this numbers game doesn't work. We've only a few Uchiha who we see take that route, of course, we've seen very few Sharingan users. We can count them on our fingers. And Tobirama wasn't fond of the Uchiha, I don't think anyone would dispute that. However, that doesn't make him a bigot, nor does it serve as some sort of counterweight to the existence of the Curse. Honestly, every single time I see the insult lobbed around, I question whether someone has actually thought it out. And it wasn't a theory only espoused by Tobirama, ages before Tobirama commented on an Uchiha in this manga, Tobi invoked the spectre of the Curse.
The issue is that Tobirama's research was incomplete and failed to take into account various other variables when coming to a conclusion. In sum he just did a shitty job testing his hypothesis and let his own bias bring him to the conclusion
he wanted, not the conclusion that was
actually correct.
Tobirama was essentially conducting an experiment when he took Madara's body, he had a hypothesis, that something was "wrong" with the Uchiha. His evidence prior to this was that the Uchiha clan when suffering loss awakened the Sharigan and thus went crazy. Problem is, we have several instances of this not being the case with our rather level headed and peaceful Uchiha members like Itachi, Kagami, and Shisui. We then have the entire warrior era Uchiha clan wanting peace and siding with Hashirama. It would seem then, that Tobirama's orginal conclusion wasn't as accurate as he thought.
Now in the context of the manga, the Sharigan is the only Kekkai Genkai awakened via emotional triggers. Why this is the case, we don't know.
Regardless, that fact alone doesn't seem to amount to some sort of "curse". It's simply how an Uchiha's unique biology works. Tobirama out of his own bias and hatred to the Uchiha labeled this biological response "a curse". Realistically, Uchiha clan members are a more evolved form of human; and their flight or fight response triggers a change in their physiology via the awakening of eye powers. In biological terms this is a form of mutation that using occurs during the evolutionary process. With the Uchiha however it's a trait that dates back to their non-human/extraterrestrial bloodline via Kaguya. Again, they have an advanced fight or flight response that normal humans lack.
This type of bodily response triggered by physically, mentally, and emotionally stressful situations is very similar in all humans. Similar combinations of chemicals are released when someone is playing sports, when they are happy, having sex, getting angry, etc. The difference with the Uchiha clan once again is that those chemical releases trigger a chakra release that awakens or advances their bloodline. How that can objectively be seen as a curse makes little sense.
We've seen various ways in which Uchiha members awaken their Sharigan.
Sasuke - when his brother murdered his clan - Hatred
Sarada - when she thought she was going to see her father - Happiness
Obito - when he thought his friends were going to die and he was fighting - Adrenal Response/Protective Instincts
Tobirama experienced these awakens via battle, and thus labeled the Uchiha crazy, because when people died they went on to kill his family members and allies....only thing is this is a normal thing we see from all characters throughout the manga. Gaara, Naruto, Zabuza, Kakashi, Tsunade, Chouji, etc. are all examples of people who got angry after particularly stressful situations in the manga and went on to pulverize their enemies. That makes them no different than the Uchiha clan yet they aren't labeled as "cursed".
Lastly, the best examples we have of "cursed" Uchiha members all have other variables that Tobirama never took into account, specifically outside forms of manipulation (Black Zetsu) and other actual curses such as Orochimaru's cursed seal. When you throw new variables into a scientific study that's already been conducted you have to redo the experiment to account for them. Tobirama's conclusion is thus outdated and very much so incorrect.
In the end, again, no actual curse exists. Instead, the Uchiha, being a little more than human, have a biological response that awakens their Sharigan. In no way shape or form can this awakening be tied to any form of insanity by the Uchiha clan members. In all instances of an Uchiha going "insane" (if you can even call it that) they were all driven by outside forces (manipulation by one or two other parties) and also suffered extreme psychological trauma in the form of physical abuse, exposure to large amounts of death and warfare, and the loss of multiple loved ones.
If anything, many Uchiha, like many shinobi, suffered from PTSD, psychosis, and depression, not a curse.
