Believe me, not only Indus and Brahmaputra that are from Tibet, some important tributary rivers that your sacred river such as Ganges also depend it on, if China seal all border water flow indirectly to your mighty polluted Ganges river, 400 millions Indians will be on panic, it require only 2% drop of the water level then India will be on total Chaos.
Err...Who is stopping you?
Instead of talking why isn't China doing it?
From your posts it is pretty clear you have absolutely zilch knowledge of river systems in general, let along India.
Psst...One of the highest rainfalls in the world happens in India...You can't stop Ganges even if you put all your might into it, not a drop of water.
Theoretically, only river you can do something about is Bramhaputra BUT even that get's a lot of water due to catchment in India.
So, not sure what you are crapping about.