A city prospers not only due to people but tangible support outside it.
For example fresh water to the 16 million is needed each and every day. Massive amounts of water needed for the industries in and around Mumbai like the Wagle Estate (Maharashtra industrial dev corp - MIDC) etc. Powai and Vihar lake are nowhere enough.
It may be inhabited(or exploited) by people from all over India - but most of the water comes from sources in Thane and Nasik districts of Maharashtra. Again it does not come from "all over India", it comes from the heart of Maharashtra. This is just one example......
The state of Maharashtra nurtures Mumbai (and Pune, and the smaller cities). Maharashtrians love our Mumbai and Pune and Thane and Nashik and Nagpur and Sholapur and all the cities of Maharashtra.