The sole reason to appreciate your currency is to boost imports in your country. Which is good if you have to rely on imports for whatever reason, like post Plaza accord Japan and post covid USA. China is in the exact opposite situation where they want to reduce imports to a minimum.
It goes beyond that.
Like I said before, the US dollar is not just used by China. Your argument would make sense in a world where only the US and China exist, but that's not the case.
If the yuan were in the dollar's position, China would gain a number of capabilities. It would, for example, be able to enact powerful sanctions regimes against actors it doesn't like - ones that could isolate and crush their economies, depending on the actor.
Taiwan could be turned into a pariah state over night, with international commerce effectively cut off. All China would need to do is indicate it won't transact with any country that does business with Taiwan. Taiwan would become like North Korea, and its pampered population would flee or riot. This same power does not exist right now, since the threat of Chinese sanctions isn't sufficient to get the rest of the world to stop doing business with Taiwan, as long as the US allows it.
Of course, these capabilities also come with costs - loss of export competitiveness prime among them, since the yuan would have to appreciate against every other currency until it is effectively over valued. The US has found loop holes around this - subsidies, unequal tariffs, and monopolistic take overs of foreign competitors - but it'd have to be targeted. You can't, fundamentally, operate an exports based economy with a reserve currency.
Historically, the US has played its role to its advantage by doing a lot of the right things you'd want to do with an over valued currency. For instance, enacting brain drains on the rest of the world, and weaponizing its consumer and financial markets to force its vassals into submission. It created powerful and vast dependency chains where governments simply could not imagine extracting themselves from the US, lest they crash their economies and are overthrown. The emphasis on "democracy" and "popular will" - supported by sophisticated intelligence and media operations - are all components of this larger strategy, and
it worked.
But the US made - or was forced to make - a key mistake: it got soft. Not in the military sense, not in the sense of being ruthless, but as the
inevitable consequence of abundance and prosperity. As they say, great times create weak men; and weak men indulge in decadence. When the entire culture of a country shifts towards decadence, it is difficult to keep
any strategy - no matter how sound - running effectively. After all, at the end of the day, policies are executed by people, and if the people executing these policies are corrupt and incompetent, then nothing can be done.
That is the situation the US finds itself today - Americans have become too complacent, ignorant, and generally incompetent to run the system their fathers created. They have become sheep, and winter is coming. It is precisely this aspect of American culture that people like Trump and Elon are trying to change. Too late, probably, but it won't stop them from trying.
All of which is to say, pretending that the US strategy is terrible and that China is right to focus on devaluing its currency - and thus the value of its people's labor - is missing the forest for the trees. China's strategy may prove to be the better strategy, long term, but the verdict is still out. End of the day, the US economy
is objectively larger based on global market value, and this allows it to do a number of things, like attract global talent and investment in a way that China never can, threaten/sanction countries into submission, gather vast quantities of vassals, buy / hollow out other countries' most valuable sectors, and out bid China in the global resources trade. But it also does come with draw backs, like not being able to run a manufacturing-based economy and creating import dependencies.