Unless there's a rock-solid alliance with the US, commercial platforms that are acquired and converted to national defense/government use can't rely on original manufacturer's repair anyway, so for the Chinese government that is within the calculations. The good news is that part and supplies for commercial jets are readily available in open markets, shopping through the national carrier.
The real concern is that chinese entering the commercial aircraft industry and stay there. learning the large scale system integration that is necessarlly for staying there.
DoD is not worrying about couple of 737s converted with a satcom dome on top. this is not game changer.
US govt (and boeing ) is worrying about 1) chinese taking away a big chunk of commercial aviation industry and hurt boeing . and 2) Chinese made commercial airliners became foundation for AWACS, Tankers, Jammers, SensorCraft, Strategic airlifters. etc. etc. Commercial airliner based platform is what has been the really THE foundation of air power projection. with out which PLAAF will always just be a bulldog on a short leash.
because, you know, with out that massive fleet of KC-135s, those B52s can't hope possiblly to attack soviet union!