Ideals
The first, most important point of them all.
What make Obito ( a Naruto wannabe ) change idea in the time span of 1 or 2 years?
The same Obito believing in heroes and giving Kakashi his famous phrase, why would he make
a speech on how the world doesn't need any heroes?.
We can easily see how Tobi and Obito share two, very different ideals, and unless he was brainwashed by someone, it would make no sense whatsoever for such a radical change.
Also, why harbour such a hate for his former sensei, his wife and his child to the point of covering the latter with paper bombs?
The same Yondaime that teached him and Kakashi about teamwork?
Remember Obito calling Kakashi captain as a sign of peace because Yondaime explained to him that Kakashi's life was the hardest of them all, asking him to understand where he comes from?
Timeline
We know the Kyuubi's attack happened 1, 2 years at best after Kakashi's Gaiden, since
Kakashi and Gai seem 14, 15 at best.
Here we see how an adult physique ( Kurenai's father ) isn't even comparable to the kids, while
here we see Tobi having the same physique as Yondaime.
Obito never was renowed to have a peculiar height or bulk ( he seems on the short side ), so it doesn't make sense for him to obtain the body of an adult in a 1-2 years span.
Still it isn't a valid point, so I'll provide others:
Do you have any idea how long I've waited for this moment?
A 16 years old saying that? After 1-2 years passed?
Are 2 years such a huge span of time to wait?
But wait, there is more:
The same Tobi that Kabuto knows not to be the real Madara saying, in front of Kabuto and Zetsu, both knowing his real identity, that
he obtained Hashirama's tissue during that battle?
Maybe I forgot something, but as I recall, Hashirama died way before Obito was even born.
But there is more still.
Tobi was the one that "created" Akatsuki, and
the one to gave Nagato the Rin'negan, the same Nagato Madara seem to know.
Wasn't Nagato, like, a lot older than Obito? He was Jiraiya's student, meaning he should have at least Yondaime's age, if not older.
Not counting also his knowledge about both Uchiha's jutsus and Uchiha's history, why would Obito have
this reaction when talking about Izuna?
It was the only reaction he had, even when talking about the Uchiha's massacre or his banishment he was impassible.
Why then responding to Izuna?
Last proof?
Madara personally knows Tobi, the same Madara that was resurrected by Edo Tensei at the same state of his death, meaning Madara likely "died" around 30-40 years of age, judging by his overall aspect.
Was Obito even born? Remember, at Madara and Hashirama's prime an old fart like Hiruzen was just a kid
Ability
We know that Obito was a funny guy, but he was no genius, hell he was pretty average.
Yet the same Obito, in 2 years, managed to:
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gain enormous knowledge on seals
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Managing to control the Kyuubi with a 3 tomoe Sharingan, a feat that only Madara Uchiha can apparently do
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Making a contract with Kyuubi
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Having knowledge, skills and abilities that no normal ninja can have, associating him to Madara once again.
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Having superior abilities than Yondaime, someone hailed as the greatest prodigy of his era, matching him in combat while controlling the Kyuubi at the same time
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Already making plans for Infinite Tsukuyomi, so gaining knowledge on Rikudou Sennin and the Juubi, knowledge being long forgotten
Seems pretty unbelievable to me.
Conclusions
Unless severe PIS were to be exploited, Tobi being Obito should be impossible, unless this is, as many before have said, Obito's lifeless body being the host of Madara's eyes, and adapted through Zetsu's substance.
Or something outrageous like time-travel, or the like.
But knowing Kishi everything is possible