Well, I do agree on the fact Buddhism being its strongest in south India during Mauryan period but not after that, it just was not at grassroots levelSimply not true. Buddhist influence in South India is tremendous. At the height of the Mauryan Empire, the south was slowly transforming itself culturally to adapt Buddhist ideologies. Go to any Southern temples and you will see Buddhist influence even today. The fall of the Mauryan Empire struck a blow to the Buddhist influence as Buddhist monasteries and monks lost their patrons. The internal fighting in Mauryan Empire caused their distant holdings to disenfranchise themselves from the capital. Not to mention I believe the central authority tried to enforce too many reforms or politically overextended themselves without using proper military force. Most likely this emboldened regions to revolt against the Empire.
The best way I can see Buddhism to propagate and replace 'Hinduism' is by taking up the social justice cause. The caste system, sati, elevation of Brahmins, occult practices, sacrificial rituals etc should be the ideological enemy of Buddhism. What Buddhism should do is to dig into the Vedic era and demolish the existing caste system that locks people into a caste by virtue of birth. To that effect, Buddhist monasteries will have to transform into an educational institution that teaches people not just Buddhist Philosophy but life skills. Also, there should be a proper separation between the Church and State. The Mauryans to my knowledge dabbled too much into Buddhism and they lost track of the military domination of their Empire.
Its not that Mauryans became too pacifistic, its that they allowed non Buddhist, especially upper class and caste of non Buddhist origin to be relatively free of scrutiny that allowed the said upper classes to attack and rebel when the Mauryans were at thier weakest, as such focus more on standardized Buddhist conversion of many such upper classes to make sure no such revolt arises in future
Essentially make Ashoka more aggressive in his conversions