I'm right now reading "The Rise and Decline of Buddhism in India" from Kanai Lal Hazra. The only book on that matter I could find in my bibliothek. As far as I have read so far, his argument seems to be that while there were never organized anti-Buddhist measures enforced, the centralized nature of Buddhism made it very vulnerable to the sporadic persecution by individual kings (he cites the examples I provided). Essentially if a ruler decided to destroy a certain Buddhist monastery it was a major blow for the organization in this area, while a similar destruction of an Hindu temple would have the same impact, as it could be rebuilt by other smaller temples still around.
Thus the removal of the key centers of Buddhism allowed the absorption of the lay people, preventing a reestablishment.
I have read a very considerable amount of literature about the disappearance of Buddhism from India, but not Hazra. I will take a look at it.
The problem with the argument that the destruction of key centres allowed reabsorption of Buddhists is with the size of the monasteries involved. The large monasteries, whose activities were recorded until the arrival of Islam, were all untouched. No one has been able to make effectively the argument that key monasteries were destroyed by `Hindu' kings. Takshashila, Nalanda, Udantapuri, Vikramashila, Nagapattana, and Jagaddala were all huge Buddhist monasteries-cum-universities, and they all flourished untouched with funding by the state (and private). None of the big ones were destroyed, or even attacked at any time by any Hindu kings. Further, the number of Buddhist monasteries was never a small number. For example, the famed opponent of Buddhism, Shashaanka of Gauda (modern Bengal), had more than a hundred Buddhist monasteries in his capital, Karnasuvarna (also from XuenXang).
The actions against Buddhist monks by non-Buddhist kings in India, where they have occurred, can be classed into two categories.
1) Individual preachers who managed to get the ear of the king and get him to remove his hated rivals. This was invariably limited to individual opponents, or small monasteries. No king ever went after the big monasteries, or after small monasteries on a large scale or even a systematic way. Consequently, the idea of Buddhism being fatally damaged by individual kings is unsubstantiated. If we take your own example and accept that Simhavishnu Pallava went after individual monasteries, it could and would have been rebuilt under his son and grandson, who, according to Xuenxang, were great patrons of Buddhist learning. The number of monks the large monastery in Nagapattana (just about two hundred miles south of where the monasteries are said to have been attacked) was churning out would make it easily possible to rebuild and re-staff those destroyed monasteries, and state funding, according to Xuenxang, was available.
2) Buddhist monasteries which got involved in politics. The example I cited, Shashaanka, went after individual Buddhist preachers and two monasteries in Kushinagara, because they helped the wife of his political opponent escape from his dungeons. The same Shashaanka, however, funded hundreds of monasteries in his capital, and there were more than twenty other monasteries functioning in Kushinagara, at that point of time, according to Xuenxang.
Post-Islamic arrival, the picture changed radically. No Indian philosophy got any state support, and central organisation, in the face of consistent persecution (especially under the Delhi sultanate and the early Islamic rule), was a significant liability. This meant that all Buddhist monasteries, which were centrally organised, unsupportable. To add to this problem was the fact that Arab and Turkish merchants replaced Indians in key trade. Mercantile support was always a significant source of income for all monasteries, and with both sources of funding gone, Buddhism simply died a slow lingering death, over the next few centuries, being absorbed into the Indian metaculture. It was not just Buddhism that suffered post-Islam. All centrally organised Indian philosophies, which required state support, or universities, vanished. Among the more prominent were the Sankhyas, the Logicians, the Legalists, Kapalikas, and Charvakists. The great tragedy post-Islam was that it greatly reduced the philosophical diversity of India. Only those philosophies which could thrive with little to no state funding survived.
The exact reverse happened in South east Asia. `Hinduism' had a great amount of penetration in south east Asia at one point of time (Khmer empire, kingdom of Champa, and the Majapahit empire were all `Hindu', in the fourteenth century). But `Hinduism' there got absorbed into `Buddhism'. Not because of any large scale persecution of Hindus, because state funded Buddhist monasteries were able to capture a larger mindshare by providing clergy for lay rituals.
Was Buddhism a different philosophy? Yes. It was recognised by everyone who knew enough to read and write. But was it a separate religion, whose followers identified themselves as `Buddhist'. No.
What exactly is the difference between a philosophy and a religion? Why are Christianity and Judaism to different religions while Buddhism is a philosophy?
I am no expert on either Christianity or Judaism, but from what I understand, the rituals of the two religions, from birth to death, are already determined and are very different from each other. The Jewish wedding/birth/death ceremonies are very different from the Christian wedding/birth/death ceremonies. This was never the case in Indic philosophies. The few differences you can find today are mostly modern. The amount of overlap between `Buddhist' and `Hindu' rituals, both personal and communal, in ancient India were huge.
The most important difference is that neither `Hinduism' nor `Buddhism' are exclusive. People were not asked to declare that they were Hindus or Buddhists. It also does not follow that if you are a Hindu, you cannot be a Buddhist, and vice versa. This is not true of either Christianity or Islam or Judaism. The lay people went to the nearest/most convenient/favourite priest when they wanted birth/marriage ceremonies. Can't get priest X, because the monastery where he lived is gone? Oh well - we'll get priest Y, who'll do the same rituals for us, albeit in a slightly different way. The rituals were still similar after all.
Same with communal rituals like harvests, spring festivals, etc. Get the most convenient priest, who will perform the rituals for you. No one cared what he believed in. Correct observation of the ritual was all that mattered.
And my point is that this different "philosophy" (if the word religion is not allowed) was removed from India where it once prospered. And that if we keep it there it would be a good start for a one world religion.
If you want to do that, you need to butterfly Islamic invasions in India away.