Why would China bother with building a new canal when there's already a perfectly usable one in Panama? China's already spent money on upgrading the port facilities at both ends of the Panama Canal; all they need to do is invest that money you're talking about into widening the locks and the Culebra Cut (and wherever else along the Canal such widening is needed) and they'd have a canal capable of handling just about any ship now in use or on the drawing boards.