The purpose is not to win the minds of Chinese citizens. There’s little that can be done to counter a lifetime of propaganda, even more so when its citizens obtain most of their information from heavily censored platforms controlled by the CCP.
Actually, that's probably the only good justification for banning WeChat. If someone's views were formed by propaganda, when you disconnect them from their reinforcement, they'll probably at least mellow with time. The point to point communications features of WeChat can mostly be replaced, but less so with the social media aspects.
IIRC, the CCP spends considerable effort trying to maintain influence over the Chinese diaspora (for instance, by getting control of foreign Chinese-language newspapers), though this effort pales in comparison to their domestic efforts.