Shri Ashwin,
I have not heard, nor read about Shirdi Sai Baba being abrogated by "brahmins". If that sort of thing has happened, then it is the age-old "trick" in hinduism. Just as the Lifebuoy soap ad says "where there is Lifebuoy, health is there!", or its equivalent in regional languages, there is in hinduism, a cliched practice of first trying to reject any NB greatness, but when facts compel, that greatness is invariably and with satellite precision, attributed to brahmin parentage (at least brahmin seed) and divine, miraculous transfer to NB surroundings. This theme is ever-recurring, from Krishna of the divine seed origin. You kindly search for Kabirdas' birth and enjoy the fun!
Shirdi Sai Baba never said anything to anybody about his caste, birth, growth or what religion he professed. He was content to be himself, in a dilapidated mandapam, smoking chilka and eating whatever food he could get by way of alms or gift or offerings. Till the very end he lived like that and that is why I hold him in high respect and view most other samiyars as fraud. As you rightly say nobody knows what he did - but during the first half of his life, roughly (not 60 years) but subsequent to that he became a source of solace for many poor and indigent people. So, naturally, his life remained essentially unchanged and unruffled. It was only during the final phase of Baba's life that he was "found out" by the rich and greedy. Yet, he did not fall for any enticements and unlike Ramana, satyasai, chinmayananda and many many more swamijis and gurujis, remained in his original mandapam only. I feel therefore that he was a person very different from all the other godmen/godwomen whom our believers eulogize today. These people were after money and comforts and were, fortuitously, successful in fooling people, and built for themselves considerable estates and riches. Thus they best exemplified the "udara nimittam bahukrita veshaH" description.
Don't judge any religion or ism from outside just on the basis of one temple and your very short acquaintance with it. All isms have their ills, just as all human bodies have very many serious diseases lurking inside each of them, but will manifest only on some occasions. In the golden temple also, if you are a sikh convert from the dalit castes, you are looked down upon. That was why even a politician was asked to clean shoes of the devotees as tankhwah, I had read somewhere. In some European country a sikh sant was shot and killed because of similar reasons. No ism is the best ism.