You underestimate the difficulty of invading across the sea.
You also probably overestimate the morale and willingness to die of Chinese conscripts.
There is a modest but very real possibility that Taiwan could fight of China by itself. I would not put money on it, but it's possible. In the long run, if China can effectively blockade Taiwan, they'd eventually win, but supplying your own military across the sea and maintaining a blockade isn't easy either.
If Vietnam joins in, the Chinese are in deep shit. If Japan or the Philippines do as well, their odds of winning are lousy. You could also potentially see Malaysia, Indonesia, and Thailand throwing in, not to mention India.
If the US joins in, China is guaranteed to lose (not least because if we jump in every other nation in SE Asia is likely to), but victory is by no means certain for them if we don't.