What downsides? Even ignoring the economic and political fallout (which could leave them entirely isolated on the world stage), they could _fail_ in their attempted invasion even if only going head to head with Taiwan. The result after both a successful and an unsuccessful invasion could very well be a collapse of the PRC. They certainly understand this. That's way they've simply used the threats as a tool of internal propaganda. They may one day go for it, but it is very far from risk-free.