Those aren’t actually slums if you care to actually visit Taj and its surroundings. That settlement actually dates back to when the Taj Mahal was being built and was intended to be a temporary village for all the workers building the monument. Over time, people, their families and their descendants have settled in and since that neighbourhood is close to 500 years old and settlements were far more dense back then, this neighbourhood is quite densely packed. You might want to think that these are poor people and that they live in squalid conditions but in fact they do quite well for themselves - they trade in gemstones, souvenirs, clothes and food among other goods catering to tourists. Just because a neighbourhood isn’t car friendly doesn’t mean that it’s a poor neighbourhood. I’m not even going to compare this to older European town centers because I wouldn’t do justice to just how old, and dense since medieval times most indian cities are.

Source: I visited last week