Metro is not a failure in the above mentioned cities. Please don't make false claims. So far 16 Indian cities are building metros/subways and another dozen cities planning for it. By 2030, more than 30 Indian cities would have operational metro. If Metro is a big failure in Mumbai, Bangalore and Chennai which are some of the primary cities of India.. why would other cities even dare to build it?
You don't need 6-7 million population for Metro. All you require is vision and money to implement that vision into reality. Metro is a Public transport. Public transportation provides personal mobility and freedom for people from every walk of life. Better and Improved public transportation means better and improved lifestyle. Not only that, every $1 invested in public transportation generates approximately $4 in economic returns. Every $1 billion invested in public transportaiton supports and creates tens of thousands of jobs.
Those figures for Chennai are very outdated and in all probability incorrect. I don't have latest data for Chennai but Mumbai urban agglomeration's PPP GDP as of 2015(?) was appx. $368 billion and per capita income was $16,881. So I reckon Mumbai metro's GDP to be around $280-290 billion and PPP per capita income even higher.
You need to show some proof, i just watched a tv debate on why Chennai Metro is lacking passengers last month and it had an expert talking about why Bangalore and Mumbai are failing. When I did google research, I came up with more evidence. Cities like Mumbai only has about 300,000 riders on travelling it daily. Even when I check instagram, most pics at Metros of Chennai and Bangalore seem to be empty.