I've spent years travelling all over India - I've eaten in five star restaurants and hotels and, more often, from street stalls. The first thing you need to understand is that 'Indian' food in Britain and elsewhere is a fiction produced by cooks from Sylhet in the hill country of Bangladesh or from Pakistan. True Indian food is very varied but, to generalise, is vegetarian and hotter in the south (and Rajasthan) than in the north. Good Indian food, as found mostly in Lucknow and Punjab is delicately spiced.
Chilis, which are responsible for the acrid hotness of so many dishe here and in India, were brought to India by the Portuguese. They are an essential ingredient of vindaloo, as much a Portuguese dish as an Indian one, but not ridiculously hot. The unique feature of a vindaloo, incidentally, is that the meat, which must be pork, is marinated in vinegar, giving the characteristic hot and sour taste. Reasonably enough, you will not find this in a restaurant run by Muslims.
Chilis seem to be used among the poorer people in the sub-continent because of a belief that they help the body to extract all the nutriments from what is often an inadequete diet. It is nothing unusual to hear middle class Indians asking in restauants whether the food contains chilis, and complaining loudly if it does.
The funny thing is that this all goes back to 18C when (British) employees of the East India Company would compete to see who could produce the hottest curry for their guests. And then we could talk about the use of the word ‘curry’ itself…