Renminbi would already be a reserve currency if they didn’t have currency controls. The reason the dollar and (to a lesser extent) euro, Swiss franc, etc. serve that role is because no one really has to worry about withdrawing their funds and converting them. China can obviously get there but that sort of trust takes time and probably other reforms in China.

For the record, I’m not talking about politics or who is good or bad here. I’m just talking about the basic reality of business transactions. Being one of the currencies accepted in global contracts requires a lot of trust.