It should be noted that the two - Rama and Krishna, were required to prove the meaningfulness of Sanatana Dharma. Kalki is mythological and Siddhartha should not be considered an incarnation of Vishnu since he founded Buddhism, which in certain sense challenges the Brahminical theories and was their competitor in ancient times. It should be noted that the notion of Brahmin as first caste should have been there since the preaching of Rama though Krishna formally formalized the caste system and should be considered the originator of Kshatriya caste. It should be noted that as per Hindu beliefs all incarnations of Vishnu must accept the dominance and prominence of Brahmins and both the historical incarnations should be considered simultaneously Brahmins and Kshatriyas. This is another reason why Siddhartha should not be considered as the incarnation of Vishnu. 

The biggest credit to Krishna is that he formalized Hinduism, theorizing and structuring in the process. The war of Mahabharat would not have that significance without Krishna and without he preaching Gita. There are various inconsistencies in the epic; like who was Karna, why did Krishna, the incarnation of Vishnu, cajole a hesitant Arjuna to kill the son of Sun-God who happens to be Vishnu’s brother, why did Duryodhana give a kingdom to Karna, why did the Pandavas listen to their widowed mother’s instruction which they interpreted as sharing Draupadi, what was the meaning of the sentence, ‘you bloody, son of blind’, which Draupadi said about Kauravas, whether the scene of disrobing of Draupadi should be considered historical in religious sense because the power-structures do not change instantly, more importantly could Draupadi be shared by Pandavas; none of the rest four could have fulfilled the conditions of Svayamvara. Didn’t it require the consent of Drupada and Bhishma, didn’t the exile amount to renouncing the claim as Pandava’s could not be compared with Rama and if exile was not sufficient then didn’t the incognito take away Pandava’s legal claim over Hastinapur? Why did Pandavas kill all Kauravas when Dhritarashtra was their guardian and Kuravas were his children? Why the name Dhriarashtra, Duryodhana and Duhsasana? Isn’t Vyas and therefore whole of MahaBharat biased towards Pandavas since beginning as Pandu’s mother enjoyed sex with Vyas and accepted Brahmins’ right to levirate, while Dhritarashtra’s mother rejected that right and also rejected Vyas’s class? Aren’t the central characters like Arjuna and Draupadi more Brahmin than what they are portrayed?

Those who say that Mahabharat is a victory of dark people over fair people are crazy communists. The North can never let dark win that too in such an ancient time. Mahabharat is the victory of Brahminism over parallel ideology and Gita is the reflection of it. Krishna let liberal, flexible, cosmopolitan, non-sectarian and believers in Brahminism, Pandavas, win over orthodox, rigid, parochial, sectarian and believers in exclusive Kshatriya dharma, Kauravas. In this sense the Mahabharat and Gita establish the permanent dominance of Brahmins over the rest of the Hindus. Gita is hardcore Brahminical preaching and even the spirituality and philosophy it teaches is Brahminical in nature.  

Krishna imparted consciousness and ego to Hindus even though Gita is against the egos and for the full surrender to the Ishvara. Without Gita, the Anglicization and Westernization to the extent observable in India in modern times would not have taken place. Of course, in the absence of Gita, Hindus would not have converted into Christianity if they were significant majority at the time of arrival of British, instead they would not have embraced its positive and required values. Without Krishna, hopes of constant incarnations and dynamism would have lost from Hindu society and Hindus could not remain majority during Islamic rule, would not have won war of relative rates of adaptability with Muslims during British rule and would not have been that ego-centric and consumerists in this integrated world.

Also, without Krishna Brahminism would not be that cohesive idea. Brahminism's most fundamental property is social order based on caste system and worship of idols. Caste provides glue to Indian society which even after almost 5000 years of existence is still so amorphous. Without caste system one can understand the fate of India; very divided and segregated if not un-united. Brahminism promotes institutionalization and homogenization of Hindu society and imparts consciousness and egos. It itself projects a kind of nationalism.

In preaching Gita, Krishna makes an error: that of preaching extreme form of asceticism, renunciation from worldly matters and he also gives overdose of spiritualism. Krishna generalizes too much and believes that the whole is sum of parts and that future can be projected from past and present. He also makes an error in assessing that in order to continue Hindu caste system, socialist and even communist principles are required. Hindus thrived maximum when they start interacting with the capitalist societies. Krishna could also have misinterpreted Sanatana Dharma with a static world and with not much economic activity. Of course, at the time of Krishna there was no major religion present around the region but he should have inferred the offshoots and opponent ideologies of Hinduism and should have taught Gita accordingly.

Sure, asceticism was necessary to keep distance with other religions. Gita did help stop the mass conversions of Hindus to other religions, particularly to Islam. Its somewhat passive and bold nature was required for Hindus to live in peace with dominant Islam in the medieval period. Hindus could not have believed in jihad, the constant revolution, and had they tried, it would have been net loss to them; in terms of demographic numbers and stability. The distributions of Hindus would have been completely different then. The beauty of Krishna is that he upholds the caste system and imparts color consciousness to Hindus. His dark color and presumably ambiguous caste was helpful in that. That’s why both fair and dark Hindus follow him.