Trying to grab Taiwan with force alone, even if formally successful, would mean losing its crown jewels forever.

As they are fully well allowed to as Taiwan is their own territory. You and I might disagree but out of the 195 countries on earth the only ones who recognize Taiwan is a country are these few:

Guatemala, Haiti, Honduras, Paraguay, Nicaragua, Belize, Saint Lucia, Saint Vincent And The Grenadines, Marshall Islands, Saint Kitts And Nevis, Palau, Tuvalu, Nauru, Vatican City.

Comparing that to the 139 countries that recognise the State of Palestine (where Israel can still do as they damn well please!) and it is quite easy to see that while some might pretend to care about Taiwan (like the US) all they really care about in the end is the money to be made from trading with/in PRC.

>China will think twice about invading.

PRC doesn't invade. It isn't the US. It gains influence in much more subtle and intelligent ways than bombing people but if they wanted to they could do to Taiwan what Israel does to Palestine and no one would do anything but talk talk talk.

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.