Answer by Uday Kale:
China may consider breaking the neck if there were any big conflicts between China and India. Their is a big presence of Indian Army in the region to prevent exactly that.
If the Siliguri Corridor were lost things would get difficult for India in the region. Due to lack of free trade agreements between India, and Nepal, Bangladesh or Bhutan, the single railway line through the corridor is the only supply route to Assam from the mainland. Even if a natural disaster were to occur 40 million people will be cutoff. Sea is not an option because no part of the cutoff land will be accessible. So in effect India will loose the Seven Sisters.
Apart from that China will have direct access to Indian heart land(Bihar and West Bengal). These is a very highly populated areas and defending them will be a nightmare for Indian Military.
In short India will loose its Himalayan buffer and China will have a platform gain substantial inroads inside India.