Naval warfare is more about hardware than manpower. American casualties in the Pacific Theater of WW2 were only ~100k dead and ~200k wounded.
The US alone would lose in that as well, because its shipbuilding capacity is minimal. But together with South Korea and Japan, it could compete against China on a level ground.
China has the ability to strike the American heartland, including naval production, in ways Japan did not.
We’re also at risk of losing strategic depth: how many more years of provocations from Washington do you think it would take a South American, Mexican or the Caribbean country to start letting Chinese drones, ships and missiles on their territory? (How confident are you in our intelligence community that this hasn’t already happened?)
The US alone would lose in that as well, because its shipbuilding capacity is minimal. But together with South Korea and Japan, it could compete against China on a level ground.