It can be $50k in India and other developing countries.
Errm...no. Using the 4% rule, that works out to living on $2k/yr. It's not going to be a comfortable life and definitely not in any of the larger cities or towns. Which is where you'd want to be if you want to have a "modern" lifestyle, with healthcare, reliable electricity, internet, running water etc.
In a city like Mumbai (admittedly, the most expensive in India probably), $12k/yr (post-tax) will just about get you into a Western-comparable lifestyle; decent-size rented apartment, access to cultural events and social life, public transit + taxis/auto-rickshaws, decent healthcare, healthy food + eat out once in a while, some domestic help etc (keep in mind I'm not including savings in this, so a local person who's still working has to earn a lot more or spend a lot less, in order to be able to save). Using the 4% rule that translates to $300k. Also keep in mind inflation in India overall is also much higher than in most Western countries. It's been in the 5-7% range for many years.
Some of the nicer himalaya hill stations seem like a better idea for a retirement destination.
If you're not retired or working remotely I would never suggest living in India or anywhere in the third world for that matter - making money is so much easier in the west.
[1] random example: https://www.google.de/maps/dir/Shimla,+Himachal+Pradesh,+Ind... almost every Indian town will have a major city within a couple hours' distance, the country is very densly populated.
I feel you. I grew up in Mumbai but I don't live there at present. The pollution seems to be worse every time I visit; I think it'll get worse before it gets better. Violent crime isn't that much of a problem in Mumbai in my experience (I've heard Delhi is pretty bad).
> Some of the nicer himalaya hill stations seem like a better idea for a retirement destination.
Can't say I haven't considered that also. Though a lot of them (eg. Manali) are horrible tourist traps. I guess it might work if you can build up a social circle there and you prefer outdoor activities (hiking, climbing, rafting) over city attractions (art, theater, restaurants and bars). And the Internet is going to be very slow. And every time you want to travel it's going to be an 8+ hour train or bus ride to the nearest major city airport. And even then $2k/yr is not going to be enough (unless you live like some of the poorer than median locals). So it's really not for everyone.
If you could see yourself retiring to a remote part of say Wyoming or Montana (comparing just the relative isolation), then you might have the right personality to make a success of it.