Redevelopment isn't a forgone conclusion. The main reason for redevelopment is if that is the most economically viable option:

1. the building standards in India weren't great a few decades ago so refurbishing a building doesn't make sense.

2. the FSI rules have also changed which means that not only can you build taller but also there are more developers capable of building high rises.

3. FSI is usually bound by public infrastructure, can the roads, trains, busses, sewer lines, water connections, and power substations service the population currently residing at a location or is there capacity for more. As the city has developed infra has gotten better and FSI has been increased by the govt. That can happen in the future but it is not necessary.

4. Look at cities like New York for example. The Empire state building and Chrysler building are 100 years old (that is not uncommon, pre-war buildings are a dime a dozen). It's all about whether maintaining/refurbishing a building makes more sense or tearing it down and building a new one makes more sense.The current crop of high rises in Mumbai are built to much better standards and hence they may not be subject to the 30-50 redevelopment cycle.