So many problems here:
- You can maintain a strategic capability without destroying your economy along the way. See for example the heavy press program. A strategic capability, very specific, hard to rebuild. You could certainly apply that to some level of local steel production (which does exist last I looked) - the way US navy ship-building competence is carefully maintained for example. You don't have to inflict that cost on most of the steel used for, say, building and bridges construction.
- You can want to run your entire economy on a "self-contained" basis but that goes against all the work that has been done for now decades in the name of economic efficiency. Such an idea will cost you dearly in the final output: standard of living. (For a small country it would be hopeless but both China and the US are large enough that they could aim for complete know-how.)
- Everyone is free to compete with anything. And yes, China is large enough that it can try and be effective at everything. Even that, does not negate the benefits of trade.
- (The US competitive on civilian vehicles?? Doesn't matter, this is a detail at this stage.)
- I don't expect very much of the US military vehicle production - in final assembly or in subsystems - comes from China. What little there is, you could import from Europe.
- The US economy "floating on military strength"??