It is a chicken and egg problem. When you have no COVID cases, the risk of taking a vaccine for older people (its side effect) may outweigh the benefits. However, once it cannot contain COVID cases anymore, the benefits/costs calculation will change drastically and they cannot get vaccinated fast enough then. It is a dilemma that the China government needs to resolve, there is no way they can continue with this zero COVID policy forever.

Why does it not? The ethics of forced vaccination notwithstanding, if China is okay with using its authoritarian rule to impose massive lockdowns repeatedly, surely it'd be comfortable using its power to force vaccination?

Because their vaccines aren’t good enough compared to mRNA. And that would show the world they aren’t a leader.

I don't understand. How would forced vaccinations make either of those situations worse?

Because it would embarrass China. Their failure would be on display for the world to see.

Why would forced vaccinations make their failure more visible?