The assumption is China will act rationally.
The current rhetoric coming from China runs contrary to that notion.
There is precedent for China invading Taiwan, that did not end well.
The Vietnamese fear of China is also based on precedent.
A Korean "Berlin Wall moment" would be unacceptable to China. The "uncertain" status of the North's leadership raises questions.
It could be any or all, depending on how they feel on the day.