In Buddha’s time his was one of numerous schools, which had a greater influence than other lesser popular ones. Even the area he travelled and preached was comparatively small (covering some five-six districts of today’s Uttar-Pradesh and Bihar states); it is doubtful that he had had a chance to preach his doctrine to the followers of orthodox Vaidic fold. One important reference of his prominent disciple Maha-Kaccana’s travel to the Surasena territory is available, There is Madhura the Raja asks Maha-Kaccana about the Brahmins’ claim of their superiority being; setto-verno, fair, pure, sons of Brahma, his own, born from his mouth, necessarily a Vaidic idea. (Jennings also notes the superiority claims of the Brahmins of Vaidic heartland area of then North-Western territory) These ideas were not that dominant in the eastern parts of the country.
Buddha’s doctrine attracted masses and scholars alike, his ethical teachings and personality must have endeared him even to the most orthodox elements as well. His choosing Pali meant that his doctrine was for all and by avoiding Sanskrit, he bypassed those metaphysical concepts and nuances with which the philosophy was heavily laden. Thus he tried to deliver his message in a fresh milieu. His moral code, unsurpassable ever since in splendor, changed the Sanatana Dharmic tradition forever. His philosophy transformed the nature and content of human thought in the sub-continent. The ethics and Ahimsa had its obvious hazards and how his school fared, when tribes with alien violent faiths marched towards sub continent, chronicles and numerous ruins from Pakistan, Afghanistan-Iran to Central Asia, stands still, in testimony.
The Sanatana Dharmic tradition comes out as a geographic and cultural concept. Though chiefly rooted in the Vedas, true to its liberal tradition the others are not excluded out of design. The Vaidic was essentially a Pravrutti Marga. There must have been in existence a strong Nivrutti Marga as well. The Shramana, Nigarantha, Vratya and also the host of schools that existed during Buddha’s time testify this. Sanatana Dharmic tradition reconciles these two paths from time to time on consonance to the Yugdharma (which take care of the socio-political progression of the society).
Krishna is the ideal Sanatana Dharma representative in the Gita. Here also lies the contribution of the seers of the land. If Buddha, Samkara, mahaveer put the Nivrutti Marga to fore, Krishna Janak and Nanak upheld the Pravrutti Marga. Samkara infused the Nivrutti Marga in the Vaidic fold is such a way that it became a part of wider fold of Sanatana Society. Though the two categories are not as divorced as they seem today, as there exist a superior objective of the experience the reality and the Yugdharma call reconciled them together.
The life of Vaidic seers was not much different from those Nivrutti Marga adherents as they too had a clear priority for the experience of reality. Also important is the reality of the world and about the ultimate reality. Both the school aims at transcending the limitation of the world; Vedanta by searching the real being who is the Kutastha, unchangeable observer of all experiences, Buddhism by examining the psychological conception of being which we experience in/by our thought process.
Ashwattha tree seems a right analogy for the sanatana dharma. A gigantic banyan tree ever growing with new branches, roots and taking nutrition from the infinity; each new sect support supplementing and in whole, to the tradition, no one conceivable in isolation; each many.. many one.. one all.. and all one.
Post Script : Though attributed to the brilliant analytical skills of Sanakara, it is no less true that there are some unequivocal assertions that falsifies this hypothesis; even if accepted the fact remains that some inherent concepts existed that allowed such potentialities which were amenable to synthesis; preponderance of reality and emphasis on its experience is also present in Upanishads. (Likewise to see the late Buddhist development – Nagarjuna’s philosophy, Zen etc. in isolation is improper. Dharma is living concept and not a static phenomenon.