The real reason to ban plastic so strictly in Mumbai is because plastic garbage chokes Mumbai's drainage systems and storm water system. And given the city's population and usage of plastic, the chokes are quite severe during the monsoon (rainy season) when it rains heavily. It is so severe that chokes can lead to extreme flooding and loss of life when it rains heavily and there is an onset of high-tide. The water sometimes has a few hours to leave the land and hence it is critical for the stormwater lines to work efficiently.
Come September this ban will be forgotten again as the monsoon season recedes. While the ban has environment benefits, "they" care little about the "environment"