We are very protective of our territorial waters, not all submarine usage is warfare.
There is no sensible equivalent retaliatory action on the part of china after such a blockade, as the us is currently food and energy self-sufficient.
Their job is to actually sink the merchant ships. The US can't do that easily with surface combatants, because getting close to a strait means compromising your position, which means risking getting suck by a Chinese hypersonic missile.
There is absolutely a sensible retaliation from China, that's to sink the US fleet in the pacific using ground-based weapons which can't be destroyed.
Hence why submarines are interesting to the US, it's the only way to force a blockade without risking destruction by very powerful Chinese anti-shipping weapons that can't be destroyed with ease.
Also, the entire blockade strategy itself is a risky gambit. China has enough stockpiled as far as oil goes to guarantee 100 days at full consumption without any imports, and before any invasion of Taiwan they would make sure to secure enough overland imports, stockpiles, and domestic production to allow for over a year of oil.
Meanwhile the political costs of blockading the Malacca Straits and co will be devastating to the US. It will be the US's Suez Canal Crisis. China will keep Taiwan anyways.
China is definitely going to break through the first island chain. 4-8 Virginia class submarines are not gonna change anything there. They are not invulnerable superweapons, and if they were the US already was going to use them.
Without air supremacy and given the massive concentration of Chinese anti-submarine assets, operating in the first island chain is a completely suicidal option.
The only possible way for this to happen is if somehow the US manages to get Russia on board with a blockade.
It might not even be necessary to work that hard at a blockade. If there's any hot action in the south china sea, most oil shippers are going to cease operations voluntarily because they don't want their ship to be accidentally shot down by either side.
>100 days
That's 3 months. Not very much.
The world is much more than the US and Canada. The EU itself would face a terrible economic crisis.
There's no way of blockading only China. The US has no way of knowing if a boat is going/coming from Cambodia, Vietnam, or even South Korea without stationing assets very close to the border with China, which we know is really not practical.
The vast majority of countries in the world will simply not care if China invades Taiwan. Why would they?
The BRI is actually seen as a positive, not negative, by a lot of people in developing countries that benefit from pretty good deals and development. China knows it needs the BRI to hedge against a US blockade and thus make it not worthwhile, so they're willing to give pretty good deals comparatively.
I assure you, the vast majority of the world will not mind. What they will mind is the US blocking off 30-50% of their foreign trade in a blockade and causing a massive economic downturn. There's no way around that.
100 days of straight reserves is a lot. If they're still importing from Russia at full capacity that's even more, around 150 days, and they would certainly increase the reserves before any kind of invasion. That's without rationing, which they certainly would do. By rationing non essential civilian use, and increasing Russian imports as well as domestic production, and increasing reserves, they would last a year. That's a full year for them to construct more domestic exploitation, more imports from Russia and Central Asia, and alternatives. The war might very well be over by then as both sides run out of precision munitions.
As far as action in the SCS, I just don't see it lasting more than a month, unless the US is trying to impose a blockade. What would the contention be once China has invaded Taiwan? There is no way the US could invade and liberate Taiwan back, so what would be the point? Just shooting at civilian ships and try not to get found from a submarine while lacking air superiority surrounded by Chinese sonobuoys and UUVs? I don't see the strategic goal that would lead to a confrontation in the SCS for months after the fall of Taiwan, nor do I see how US surface ships could survive.
As for companies not wanting to do it, have you considered that many of those companies are Chinese? What do you expect them to do, nothing? Some are even Chinese state-owned enterprises.
I could even see the PLA making these ships unmanned as bait in the SCS to get submarines to shoot at them with passive sonar rigged in unbeknownst to the submarine, triggering a launch of China's air-launched anti-submarine torpedoes.