Further, the US is highly unlikely to be dumb enough to fight a war in China or Russia. That would require ten million soldiers just to get started anyway. It simply isn't going to happen, any more than Russia or China are going to cross the Pacific or Atlantic and invade the US. China has very little capability to project outside its borders, and is in the process of modernizing its military and shrinking its forces. Russia has zero capability of projecting serious conventional might outside of its borders, or those extremely close by.

So what war of attrition is likely to occur? Whatever that is, it'll be fought closer to the enemy's backyard, than the US backyard. The US has military bases all around the planet, nobody else does. So the US can strike, perpetually, at foreign targets via force projection, while limiting damage to its domestic manufacturing capabilities. Said manufacturing capabilities would be, much like WW2, ramped up to meet demand. The US has a massive labor pool, and if desperately needed, could grant citizenship to tens of millions from Latin America in exchange for building armaments in factories. Unlike China, the US is capable of 100% domestic commodity supply, and if pressed can also fulfill all of its own manufacturing and technology needs (which China can as well, but they don't have the domestic commodities the US possesses).

To win a war of attrition, in a conventional war, the US could simply spam the construction of new B-52s and bombs, and then leverage its global bases, pressing an unrelenting bombing campaign taking out all of the enemy's manufacturing capabilities. No present or likely future enemies of the US could strike back at the US domestically for any sustained period of time.

Now, if we're talking China in 30 years, will they be able to project across the Pacific all the way to Hawaii and California? Interesting question, it's going to depend on whether their economy melts down due to the hyper debt accumulation they're undergoing now, and whether they end up in perpetual debt stagnation like Japan (while their demographics melt simultaneously, and the demand for entitlements to take care of the elderly soars, sapping their financial capabilities even further).