I'd heard previously that Buddhism used to be a very dominant belief system in India, but at some point the Buddhist community in India markedly shrank. This seems unusual to me, as some of Buddhism's holiest sites are in India and it's the place where the religion was born. The only reason I was given was that a series of Hindu thinkers provided a number of arguments that served to convince Buddhists that they were wrong, but this doesn't strike me as a satisfactory explanation, because I don't think communities are, in general, the sort of things that can be "convinced" like individual people, and because the other examples of religious disappearance I can think of are largely due to policy prescriptions, persecution, or the aftereffects of historical events.