> Taiwan's defenses can hold up against minor probing from China, nothing truly sustained.
It's the same for every small country bordering a potentially hostile much larger neighbor. Nobody excepts the small country to be able to withstand a full-scale invasion. The point is to make that invasion expensive enough to not be worth it.
You're not even wrong. With Taiwan being an island, any prospects of invasion are going to be bloody. Successful marine invasion are surprisingly rare in history. Usually, they are only successful because the defender could not set up a defense, or the attacker committed overwhelming resources. However, Taiwan's strategic position (it guards access to the mainland from the ocean) and the ideological accomplishment of having tied up this loose end might make it worth it. And in the event Taiwanese economy and its importance declines, it could become harder for the US to justify defending it.
The true deterrent to China isn't any treaty agreement to protect Taiwan, which doesn't exist. It's the realpolitik of 30,000 US troops in Taiwan.
Any significant & sustained attack against Taiwan would harm US troops. They'd be little more than a speedbump for China if China acted quickly & in force, but that speedbump would require a significant-- and perhaps disproportionate-- response from the US.