China accounting for a majority of production in an industry doesn't prevent the US from sanctioning China; if anything, it encourages the US to play on the fear of Chinese monopoly, which leads to even more sanctions. The reason is the same, ironically, on both sides - it's to force the domestic industry to diversify their supply lines and dependencies. Politics dictate economics and always have.
Also, to bargain, you have to have the right leverage. The problem with China's bargaining power vis-a-vis the West is that the West historically owned the high technology, while China just owned the labor and infrastructure. The latter cannot function without the former, but the former can function without the latter, since the West can take their technology to, say, India or Vietnam or Indonesia, and manufacture there.
This isn't to say those countries can fully replace China, but rather that scarcity is a natural law. If what you own is scarce (and high technologies like EUV are scarce), then you have more leverage. By contrast, if what you own is abundant (and labor & infrastructure are abundant), then you have less leverage. If China was the only major source of labor and infrastructure in the world, the West wouldn't dare to sanction China; but because of the pyramid effect in which there are much more competitors at China's level, than at the West's, the West believes it has the bargaining advantage and thus, the "right" to sanction China.
In other words, they are trying to show China how "abundant" labor and infrastructure is - by forcing it out of China and in doing so, reducing China's leverage.
The proper response is to show the West that their own "scarce resource" isn't nearly as scarce as they think - by developing alternatives to Western technology that, essentially, reduces the scarcity of high technology and thus Western leverage.