It was, for a time. https://en.m.wikipedia.org/wiki/Decline_of_Buddhism_in_the_I...

Buddhism was prominent in India till Muslim invaders came and conquered India. Buddhist Monastery were sitting ducks for them, Buddhism was eliminated in the first wave of Islamic invaders.

Of-course there were struggles with others before that, such as Brahmins, but those were political struggles, not existential. Many times Buddhists gained the favor of Kings, other times Brahmins.

https://en.wikipedia.org/wiki/Persecution_of_Buddhists

Yes, Buddhism cannot be tolerated by Brahmanism. Its simply outside the caste system that removes power from Brahmins.

PS: https://en.wikipedia.org/wiki/Persecution_of_Buddhists

EDIT: https://medium.com/@ArainGang/hindu-persecution-of-buddhists...