Huh? China has about five times the population of the USA.
First, not all that many "small AM station" engineers have the gear (an OIB, for example) to rig an inverted L antenna.
But more important, just as many AM stations are in rural locations as FMs. In fact, far more FMs are in central city locations (Hancock, Willis, ESB, etc.) than AM stations. AM's are often on the lowest, most flood prone land and can be difficult to reach in major storms.
If anything, I'd say it's a toss up since AM's and FM's can be equally accessible or inaccessible.
The large public companies have all taken impairment charges on their AM stations, so the asset values are nowhere near the level they might have been at a decade or so ago. The investment in HD for AM's in major markets is a very small figure, and likely has been fully amortized already.
As long as an AM is making money or serves a significant strategic function in a local cluster, none of the major operators will be shutting of a KFI or WiNS or KLBJ or WSB. But when the station is of no value and can never be made profitable, it will go away, either by a sale or closure (think 1310 in Dearborn, MI or WQEW in New York).