One of the few benefits of being old is that most things have been seen before and therefor one realizes that many worries are misplaced. In the 1930s when I was a kid, the people in the Armed Forces were virtual oddities to most Americans. The two societies were quite distinct and had little in common on the surface -- yet, those serving came from that greater civil society and reflected it quite well. WW II of course changed that -- not necessarily forever...

I think JW is correct in ascribing some of the current angst on that score to the ascendancy in Academia of the anti-everything crowd from the 60s; most of them do not understand the Beast and it therefor worries them; all they know is that they don't like what it is or does...

They have transmitted that worry to the ever larger population of tertiary students. It has always fascinated me that coterie is first to call for some form of citizen service -- explicitly including the military for some -- but themselves would (did?) go to great lengths to avoid such service. Most would go to equally great lengths to insure that if their children had to serve, it would not be in uniform. I think there's some incongruity there...

In any event, JW is correct when he notes that a civilian - military disconnect is the norm in the US. I served during a period when one could wear a uniform anywhere and also later when one was ill advised to wear that uniform away from the base or post. I've been insulted, had things thrown at me and been subjected to petty tirades by ill-informed people half way around the world and back. No big thing, one simply considers the source and moves on. Yet, in all that time and since, the Armed forces of the US were and are today nothing more or less than a broad reflection of the society from which they spring -- with the minor exception of the presumed elite other than in exceptional cases.

Thus I think that your statement
"...When citizenship no longer requires defense of the society and the defense of that society is in the hands of people who have no vested interest in that society...
reverses the problem; it seems to me we should be worried when the defense of the society is in the hands of people who have a very strongly vested interest in that society -- because in any democracy it is after all their society, is from where they come and is the home of their friends and relatives and is the place to which most will return (and that is emphatically the case now) -- but that society has little or no real interest in they who would defend it.