Even in the 1800s, deflation was not necessarily bad for the economy. . This prolonged period of persistent deflation is exactly when the US grew from an relative economic backwater to become the most dominant industrial power in the world.
Agreed, deflation is not necessarily the problem. The problem is low income and consumption growth which is running at~3% currently. In fact the nominal gdp growth rate is around 3% and is getting a push from deflation which adds to get the real growths at 5%.
There would be no issue if the wages were growing, consumption increasing, and employment rising. But that is not the case, the economy is growing at around 3-4% nominal rates, which is too low for this stage of development.
Ultimately, nominal GDP (in USD) has been stagnant since 2021. This is somewhat like Japan, whose nominal GDP (USD) has been stagnant, or even declined, for the past 3 decades.
The currency appreciating is the solution, it's in fact required. A 20% appreciation in yuan, will increase GDP by 25%, bringing it to around 25 trillion USD.

