Europe banned the Antonov AN12 on safety grounds. China could use a similar argument, insisting that the MAX meet modern safety standards rather than those it was certified under back in the 1960s. This would be impractical as major redesign work would be needed.
The COMAC C919 is a modern design with considerable western content, unless significant safety issues come to light after it enters service, it would be difficult to find a legitimate reason to ban the type. Even if it was banned from US airspace, most of its customers wouldn’t be flying it there anyway. Having the long haul C929 banned would be a different matter.