China is getting more and more assertive in the region, and the only reason they haven't achieved hegemony in the Pacific is because of the US. If we appear to waver, or to lose interest, there's nothing anyone else can do to stop China from doing anything else they want to do in that area. And everyone knows it. That's one of the reasons the US and Viet Nam are getting all chummy these days. Same thing with Philippines. It wouldn't surprise me to see a lot of other regional players, including ones we don't traditionally think of as partners, climbing on board as well.
I admire the Taiwanese but they would get crushed. Their coastal defenses are not particularly useful against a force that can go over them by air, or through them, or simply bypass them by going upriver in hovercraft. I believe the Taiwanese would fight, but they have no staying power. China's also not particularly concerned with human rights, and there are literally no neighboring coutries to the island of Taiwan, so there is no ability to easily flow in troops or supplies, and no Western-style governments worried about what the troops on the ground are doing in order to secure the populace. They would probably also throw whole bunch of economic development and social services at the population and try to keep things as close to status quo ante as they could in order to tamp down resistance.
The population of Taiwan is approximately 23.5 million. That's considerably smaller than both Iraq and Afghanistan. It's also considerably closer, in terms of distance, culture, and language. For many reasons, the issues we had with Iraq and AFG simply won't be at play with China/Taiwan.