The problem is not about the votes. The problem is that the HK people have spoken. And they are supporting the rioters which gave the rioters legitimacy. That would embolden and encourage the rioters to take extreme measures. Either the central government caves or they would face months or even years of unrest in HK.
It doesn't matter. If they take extreme measures, it will detract the movement from HK citizens. The West won't be able to keep calling it peaceful (but they will still blame the central govt.) and the final point, if their extreme measures includes "actual terrorism", central govt. has no choice but to step in.
And once they do, HK's autonomy is no more. Forget 2047 if that happens, it will happen soon.
The other scenario is months of protest. Central govt. just has to sit back and let it be. The city is in a recession. It will be a long, deep one if protest continues. And Beijing bailing HK out means it will come with preconditions. More tight controls on things, less autonomy.
Time is on Beijing's side. Years of unrest will do nothing but ruin their own city which will then be bailed out by the mainland.