STRATFOR:
"Because India's population will surpass that of China in 2030 or so, even as India's population will get gray at a slower rate than that of China, India may in relative terms have a brighter future. As inefficient as India's democratic system is, it does not face a fundamental problem of legitimacy like China's authoritarian system very well might."
-- R. Kaplan
Brazil GDP: 2.5T (2012) Russia GDP: 2.1T (2012)
India GDP: 1.9T (2012)
India isn't even in the same league as China. India has to compete with Russia/Brazil. BRIC is an old grouping from years ago.
Having a huge population and being democratic doesn't automatically give you a bigger economy. Look at South Korea (1.3T) or Japan (6T) comparatively.
Also, read the CIA world fact book pages for China/India.
If such a scenario were to happen... it'd would be pretty catastrophic the world over. GIven how tied the US and China's economies are tied, I don't think the US would allow a USSR style collapse... :\