Traditionally nothing. However Taiwan became the centre of semi-conductor manufacturing and research such that it is now crucial to the national security of almost every nation, not just the US.
However the real reason why Taiwan can't either fall to China militarily or re-unite with China peacefully is that Taiwan forms the centre of the "Island Chain Strategy" which is a containment strategy established by the US after the PRC came to power in the Chinese mainland and ROC was relegated to Taiwan.
The Island Chain serves to contain the PLA Navy such that China can't operate as a Blue Ocean navy, i.e operate in international seas/oceans. Additionally it sets up a small number of chokepoints that can be used to completely isolate shipping in/out of China so they will alway be able to apply economic pressure through blockade.
This is the -real- source of tension between China and the "West" (even though it's really just the US/Australia/Japan in this case, China is relatively friendly with European countries etc).
If China was able to do the same thing to the US you can imagine the US would be pretty uncomfortable with that situation too.
Without being able to operate freely in the Pacific China's own nuclear deterrence is less effective as they aren't able to move nuclear ballistic subs without detection outside of the containment. This generally means needing bigger, more capable (read MIRV) ICBMs. Also generally means development of containment busting weapons, namely hypersonic nuclear tipped carrier-battle-group destroying missiles. They need effective nuclear deterrence to ensure their nuclear capability can't be disabled in a first strike. Specifically because their main adversaries are the US and Russia (yes, Russia is traditionally a Chinese adversary) both of which have significant nuclear assets so they need their own to ensure MAD is in place.
TLDR: If Taiwan was to come under Chinese control either by force or peacefully it would break a decades long containment strategy by the US, securing Chinese access to the Pacific for both trade and the PLAN.
However the real reason why Taiwan can't either fall to China militarily or re-unite with China peacefully is that Taiwan forms the centre of the "Island Chain Strategy" which is a containment strategy established by the US after the PRC came to power in the Chinese mainland and ROC was relegated to Taiwan.
The Island Chain serves to contain the PLA Navy such that China can't operate as a Blue Ocean navy, i.e operate in international seas/oceans. Additionally it sets up a small number of chokepoints that can be used to completely isolate shipping in/out of China so they will alway be able to apply economic pressure through blockade.
This is the -real- source of tension between China and the "West" (even though it's really just the US/Australia/Japan in this case, China is relatively friendly with European countries etc).
If China was able to do the same thing to the US you can imagine the US would be pretty uncomfortable with that situation too.
Without being able to operate freely in the Pacific China's own nuclear deterrence is less effective as they aren't able to move nuclear ballistic subs without detection outside of the containment. This generally means needing bigger, more capable (read MIRV) ICBMs. Also generally means development of containment busting weapons, namely hypersonic nuclear tipped carrier-battle-group destroying missiles. They need effective nuclear deterrence to ensure their nuclear capability can't be disabled in a first strike. Specifically because their main adversaries are the US and Russia (yes, Russia is traditionally a Chinese adversary) both of which have significant nuclear assets so they need their own to ensure MAD is in place.
TLDR: If Taiwan was to come under Chinese control either by force or peacefully it would break a decades long containment strategy by the US, securing Chinese access to the Pacific for both trade and the PLAN.