It’s not just about stability (and rmb is basically pegged to dollar anyway) - you also need to run a deficit and that’s not something China is willing or able to do for foreseeable future
At a guess...being the worlds reserve currency creates demand for your currency which drives it's price up. This is bad in general because it means your exports become more expensive. The way you deal with that is to create debt to export in the form of US treasuries, which foreigners can then buy. One way to create treasuries is to create a public deficit (not strictly necessarily, you could raise money even with a surplus and then invest it)
Why is this needed?