I have heard this argument several times whenever the topic of economic sleeping accommodation comes up, and it begs a question- if for once I accept your point that the expectations of Americans of privacy and personal space are greater than Europeans and Asians (?) and they would never accept sleeping in any thing that is less than a private room, so does that mean people in America never travel by planes? All those transcontinental and international flights with Business Class and First Class must be running so empty out of America, no? I wonder why there are so many international airports and airplanes in this country then.
I've traveled in shared compartments a few dozen times, mostly in Eastern Europe, and I've never had a problem with it. The issue I think that you would have in the U.S. is with strangers, especially of different genders, sharing compartments with doors that close. Questions of theft, harassment, and assault come up, when I mention such accommodations to Americans. Has any U.S. railroad, even in less litigious times, ever offered sleeper service where strangers shared a compartment?
Actually if you look at the situation in India, just as a random example (the situation is similar in most of Southeast Asia) the only compartment accommodation where doors close is AC First Class, and IR takes extreme care in assigning accommodation making sure that Indian sensibilities about separation of sexes, which is way way more strict than in the US, is adhered to strictly.
All other sleeping accommodation is in open section cubicles with no doors, and depending on the class of travel curtains to give some privacy in ones berth, or not. Theft is always a possibility and most who have such concerns carry along a bicycle chain and lock to lock their baggage to a fixed post under the seat or in the luggage rack. But broadly speaking, the risk of theft is no more in an open section than in Coach.
Lie-flat chairs, on the other, hand, make a lot of sense for coach. Let's remember that Amtrak long-distance coaches were designed in the 70s, and reflect standards of those time. I only traveled in first class internationally a couple of times in the 80s and early 90s, but what I remember (through the haze of champagne, caviar, and 25-year-old scotch) is seats that were broadly similar to what Amtrak Superliner coaches have. When did lie-flat chairs come into service on airliners?
The first time I flew in one was about 10 or so years back in Sept 2001 one week after 9/11 on BA in Business Class (when I had to make an emergency trip to India to attend to my Dad's illness). So it might have come in a little before that, though the US airlines were late getting into that game.
One concern I'd have, though, is that Amtrak has enough trouble keeping their present chairs in service. How much harder would it be to maintain a lie-flat one?
Yes that would be a concern.
I happened to be on Amtrak today and had a measuring device with me, so I took a few rough measurements. It looks like in a regular 85 footer with single vestibule, an ADA restroom and a regular restroom like on Amfleet 2, you can find room for 10 rows of 72" pitch lie flat seats without doing any herringbone. Which means you can fit 40 passengers. This would imply that all else being equal, if you do not also burden the passengers with god knows how much extra for meals and premium service, you could get away with charging a little less than double the coach fare to come out even. It would be considerably less than double if compared with a 66 seater. The cost of service would depend on whether some bedding is provided or not.
To provide a slightly broader berth in the sleeping position one could go with 60" herringbone 3 abreast with effective bed length of 72" to accommodate 36. The only difference will be about 4 to 6" wider sleeping space above ones knees, and a quite a bit narrower below.
In either arrangement luggage space would be exactly the same as in Coach. In classic sections which would have similar capacity there was a problem with luggage space because the ample overhead storage space was not available, being occupied by the upper berth.
But as Anderson rightly points out, these seats are more complex pieces of machinery, though I hear that they are very robust, modular, reliable and relatively light for what they do, and do not require a lot of maintenance. I suppose technology has come some ways in the area of electrical servo mechanisms.