Let me start by welcoming @lemouth back. Contrary to @gentleshaid opinion, I knew you weren't kidnapped 😁

I was not even really away ;)

From the links you embedded in the post I discovered that the LHC CERN is not even up to 10years old. (10th Sept 2008)

It depends how you count. If you count from the first mention of it in 1986, it is kind of older ;)

Back to whether China should build the great Collider or not?.

This is at the end of the day politics... The targeted time is in any case in the post-LHC era.

From another angle, will there be any seismic impact if two collider are placed within same geographical location?

At CERN, the LHC will play the role of a pre-accelerator (you can check this on the map). So, the two colliders will interact with each other, but on a controlled way. But no seismic impact, except maybe real earthquakes that won't probably hurt :)