Why? One Road One Belt has been the core of Chinese national strategy for 10 years and it will be for next few decades. With an India's dominated Indian Ocean and subcontinent site in the middle, their access to Europe/Africa/Mideast market & natural resources will be living under India's mercy. Not only them, but the whole Asia will have the same problem.But if that happens, we'd have more chips on the negotiation table. Even though less likely, China itself might want to simmer down tensions with us temporarily in exchange for a stable Subcontinent so that they can focus on US.
The Americans will have their issues too: once Pakistan is down, who is going to stop India from controlling middle east and half Africa? The challenge that India will then bring to them will be much bigger than China.
Basically, you can treat India's geographic location in world island as the same as Germany in Europe.
So, the most possible result will be: US and China (maybe plus Europe) will put their dispute aside, and come together to fight India.