Sacrificing a couple of rooms for an elevator and just adding extra cars may or may not be a good idea. If the aisles can't be made wide enough for someone in a wheel chair to travel around the train, having the elevator would be a waste of space since that would be the only place they could go. If wheelchairs could be accommodated, then my question would be do the modifications sacrifice any existing comfort in a noticeable way? As for the cost, adding cars could mean adding engines which adds costs that may or may not be covered by the extra riders. Just because something isn't meant to make a profit, doesn't mean they should run up their costs without thinking something through.
I'd have a problem with using multilevel sleeper cars in the US because there is an expectation that you will move around the train at some point during your trip. On most European trains, what night trains they have left generally run at times where the only think you should be doing is sleeping since the run times are like 10pm to 7am. I've also seen a few trip reports on YouTube and some companies actually lock the doors between cars so only the crew can travel between cars. If you're on OBB for instance, their trains don't usually have a dining car, so there is no real reason for you to leave your car once you've found it.
Uh huh! And if you're in a roomette and must get up in the middle of the night... you must go to the very overused upstairs WC which frequently clogs up and needs to be shut down... or circumnavigate that tight stairwell to use the downstairs facilities... for a very unpleasant midnight trek.