Where did you get this from? I am Indian and this is the first time I'm hearing of it.
The one child policy means there are no people to do the manufacturing in China in the future. The population pyramid is inverted. You can't do manufacturing without lots and lots of people. Or at least you'll get out competed by your neighbours who have lots of labor.
China invested heavily in machinery and automation many decades ago. They'll be fairly fine.
I visited India a few months ago. Loved it. But I saw construction being done with donkeys hauling dirt and people shoveling the loads onto the donkey's cart with tiny hand shovels. Scaling up from that to the degree of manufacturing powerhouses like China have is not going to be easy.
It also means they no longer do the cheap high volume manufacturing, which will go elsewhere where the cheap labor is.
This is how it goes. Manufacturing keeps migrating to cheap labor. China is running out of cheap labor and volume manufacturing will find it in East Asia and India and Africa instead, because these places will still have lots and lots of young people while those in China have aged out.
China's rise was due to smart planning and investment in their people and infrastructure. The country went from peasant farmers to the manufacturing hub of the world in a generation. Now they're in the process of becoming a major hub of knowledge workers while keeping a strong manufacturing backbone.
And one thing I rarely see mentioned is that the Chinese government keeps its currency artificially devalued in order to sell their products cheaply and undercut competition. Looking at their economy purely on a GDP basis really underestimates how big their economy is. GDP by PPP is more accurate. If China ever decides to stop loosely pegging its currency and let the value naturally rise, their GDP will very likely swell, since GDP is measured in US dollars.