As I explained, the spread of Vedic culture went hand-in-hand with the caste system, which is custom made to give the ruling elite certain advantages over the masses. Now, if people want to believe that the masses willingly adopted castes which restricted their social and economic mobility, as well as their intellectual freedom, then I don't know what to say.
The likely reality is that the caste system had to be forced onto the people by their rulers. Ergo, the Vedic culture had to be forced.
It is worth noting that, as soon as they had the ability to exit the belief system which imposed the shackles of rigid casteism upon them, the lower castes voted themselves out of Hinduism and into Islam or Christianity.