Curious to know how it could have flourished as it did throughout India, spread across Southeast and Eastern Asia where it laid down enduring roots, but in India it culturally went extinct.
I recall that Buddhists in India were persecuted, with some prominent leaders having been killed. So I suppose a second question is why were Buddhists seen as such a threat to those in power?
There is not the slightest hint of any evidence to support the actual persecution of Buddhists or the destruction of monasteries as such by Vedics.
The account of Pushyamitra Shunga attacking the Buddhists is a myth based on the document of Ashokavadana, the purpose of which was only to glorify the reign of Ashoka and isn't reliable as a source of history.
Rhys Davids had already brought out the source of this fabrication and as to how it gained importance and dismissed it in his book, Buddhist India.
BM Barua, an Indic Buddhist scholar rightly references this in his book as well and presents the studies of other scholars on this important question as well. None of them have accepted the supposedly heinous acts by the Shungas. Search Pages 20-23
https://rarebooksocietyofindia.org/book_archive/196174216674_10153039333031675.pdf
However, there is ample literature to show that both sects made disparaging remarks and attempts to wrongly portray deities of that time. Bhattacharya's book on Buddhist Iconography.
https://archive.org/details/indianbuddhistic033312mbp/page/388/mode/2up
The scope for speculation and interpretation was always an integral part of spiritual discourse and so the Shakyamuni Buddha's dismissal of caste and the obsession for maintaining purity by Vedics pointed merely to sectarian social biases that never resulted in heinous crimes of any sort. The inherent nature of the Dharmic tradition lies in debate, and it was the prevalent method in those times for the loser to acknowledge and follow the tenets of the winner after the debate. Due to similarities in the basic philosophy, there might have been assimilation because of the segregation of Varnas among different philosophical systems and a few reverting back to it. Each future philosopher was better prepared to present their interpretation of the Truth, thanks to the previous debates.
TRV Murti affirms this in his book as to how this natural evolution of philosophy is the characteristic of the Dharmic tradition.
RC Mitra has done extensive research on how there was no such persecution by Hindus at any point of time and he traces the extent of Buddhism is different states. AL Basham points this out as well and critiques him in another places but agrees with this conclusion. Full paper seems unavailable.
As to what was the clincher in its decline, even Dr BR Ambedkar in his writings provides many sources and opines that it was the Muslim invasion of India that caused not just the downfall and destruction of Buddhism and its monasteries, but also attempted to destroy the fabric of Dharma itself.
Lars Fogelin, an archaeologist, presents the growing difference among lay persons and the over-reliance on the Stupas by Sangha to estbalish authority might have weakened its prospects for longevity, and that helped the Vedics in some way and that it was a gradual process and the decline was confirmed when the Turkic invasions destroyed the monasteries.
So what you're suggesting is starkly opposite to the epitome of intellectual thought during an unparalleled era of knowledge where as the other part of the world was haggling over intolerant conversions and what not.