Brumby
Major
The problem is that we don't know how things might spiral out of control once unregulated universal sufferage is implemented. Oh sure, Beijing could absolutely intervene if things reach a critical mass -- but it's better for them to simply prevent such a situation from arising in the first place.
I do not agree with the term unregulated universal suffrage because the issue is not with regulation but the tightness of it that makes the term a sham when discussing HK. The Basic Law is clear on the process to be followed on how the CE is to be selected and so we need to abide by the law and that means certain regulation needs to be followed. Why it is a sham and doesn't reflect the spirit of universal suffrage I have mentioned on it before and I would not labour on it anymore.
Now this would be an entirely different story if the protesters had made it clear that they had no intention to even think about secession or challenging Beijing's rule on the mainland (i.e.: if they showed their demands would not oppose Beijing's core interests), while also stoutly refusing foreign (US, UK specifically) help and funding and rejecting foreign support so as to convince Beijing that they were not influenced by other countries -- but obviously the protesters and the movement overall has acted against these listed points for potential reconciliation.
The problem I have with your reasoning is that secession challenges has to come from the people and to be even remotely successful you need a vast majority of them to be on your side. The role of a CE (and universal suffrage) is really irrelevant in the equation unless you are talking about subversion but clearly Beijing has recourse in such a situation.