My impression from reading the authors that I've read Is that the Mahayana and traditional Buddhists were one Sangha because they had the same Vows. There already was a proliferation of a score of various views, many of which had their own unique non-Mahayana suttas, suttas not recognized by their fellow monks. We today don't recognize the already present traditional suttas diversity may have masked the presence of Mahayana sutras back then. Why? Because we are over-aware today of the big distinction of Mahayana sutras and the Buddhist canon we have been brought up to think about. Back then if you observed the same conduct and followed the same practices except for in your spare time and maybe with a study group that had its own unique sutra, the fact that you had a different opinion and the Sutra of your own wouldn't have stood out quite the way we would think today. Apparently this went on with until the 3rd century. It may be that it was the translators who rounded up and packaged sutras for China did something that was rather unique and bundled them all together so they could be looked at as a kind of genre, whereas in India local monasteries basically had maybe one or 2 new sutras and groups that promoted them, and it took time for a mutual rejection feeling to build up "as the rule". Apparently Nagarjuna was operating against the backdrop in the 2nd century where the Mahayana wasn't a big issue so much as it was nonthreatening or ineffective within the larger Buddhist communities around it. It may be that it was with the Yogacara sutras and their effort to reintegrate the foundational Buddhist teachings and practices into the big picture of Buddhism that made Mahayana Buddhism bloom a little in India. But I don't see Mahayana Buddhism taking India by storm. It may have been in China where it really developed a public face first. This is just the impression I have come away with.