I would agree that most ship plans are as you say, more like cruise ship cabins. I disagree that Traveller can't take a cue from the old train berths.
A lot depends on how much demand there is for 'personal space' vs. shared amenities. If towards the latter, buying back space in personal/sleep quarters frees more up for featured uses.
Another concept I've been working up is the 'amenity stateroom'. The idea is that the stateroom comes with an additional feature not normally found- a hobby workstation with tools, a workout device, a musical instrument, a greenhouse, specialized pet facility, virtual entertainment system, dedicated stewardbot, personal kitchen, extreme enviornmental support, etc.
In most cases this would take up extra room, so a Pullman berth type system would help get back space to allow for the amenity.
From a ship's plan perspective, amenity staterooms probably wouldn't end up like the uniform cubby holes they are now. There would definitely be a class of passenger that would not care for a 'socialize first' design scheme and want their private space, so you would probably have a mix of amenity staterooms, regular open privacy staterooms, and half size staterooms that allow for more common area.
As to who gets what room, that would be on a first pay first choice system- with the exception that the high passage passenger could bump a 'reserved' middle passage out of their previous preference. Potential passenger drama over such bumps....
Private kitchen rooms would be a premium for middle passengers.
I was considering adding in a cultural interstellar travel custom where passengers with amenity staterooms are expected to provide some entertainment/food/open house 'event' surrounding the amenity, just to break up the monotony. Just an hour or two, but makes for a different trip due to quirks in both the ship masters' amenity choices and what the passenger does with them.
One other thought while I was ruminating on all this and how one of those single steward ships would actually operate.
The steward has waited on high passengers 24/7 for an entire week. The steward probably also does things like order local source gourmet food during the week down. They need off time probably more then any other crew member.
The last thing they are going to want to do during their 'off week' is scrub down the staterooms to a hygiene level like regulations and high passage passengers would expect.
So I'm thinking that part of the 'Life Support' expense is starport service personnel cleaning out each stateroom and common area.
Interesting things could develop if crew members refuse the service and smells not to mention hygiene threats develop.
The stateroom service could also be a vector for devices to be installed towards nefarious ends, and smuggling small items hidden in crevices between starport crews- possibly to be discovered by passengers, or worse illegal items in crew quarters during a ship inspection.