I posted this in another forum. Some more info if you are interested:
A bit of background about Indian football.
The original Indian league system has the I-league as the top flight. The I-league contains most of the old, existing clubs. Some like Mohun Bagan are a century old and have huge local fan bases and "history". Unfortunately for a long period of time, grassroot football interest was limited to a few states with most of us plastics coming from big cities, which generally don't have I-league clubs or even second division clubs.
In 2014, with full backing of AIFF (the Indian FA), the richest guy in India (Ambani) created a MLS/IPL style franchise league, all with media hype and glamour. It's a closed league without any promotion/relegation or any relation with the original league system. Mumbai city fc was created back then as a franchise. There has been a long messy battle between the I-league and ISL since I-league still is the official league and gets Asian CL and Asian cup qualification since the last 5 years. It's only this year, that the AIFF decided to side with the ISL and make it the top flight but it's still a closed league. ISL's money and it's marketing are the reason why football has seen a boom in India lately but the I-league clubs are the original clubs and rightfully annoyed to be suddenly relegated to the second tier with no prospect of promotion. It's likely that there will be long legal battle about this, but ultimately the richer league will win, I think.
If you want to think of a parallel for this league mess, think of the ECB creating the "Hundred" instead of using the existing county cricket teams.
As for Mumbai city, it's great news. ISL clubs are not immune from going bankrupt. My own "local club" of sorts when down this year.
https://en.wikipedia.org/wiki/FC_Pune_City. I doubt CFG will abandon any of it's clubs, so it makes it a lot more resilient against financial woes. The ground is tiny and shit though.
https://en.wikipedia.org/wiki/Mumbai_Football_Arena . Mumbai city fc is also a middling team. Some of the other teams in the ISL have larger fan bases or simply better players/coaches.