There are small elevators that could fill the bill on the market already, but any kind of mechanical or electrical failure could leave a vulnerable person trapped for a long time in many areas. Not sure that would fly. You would still need the stairs for safety, so you would be taking space away from something else. No reason to get wheelchairs to the upper level in the sleepers, they won't fit in the aisles.
But just because someone normally needs to use a wheelchair does not prevent them from flying merely because of the difficulty in evacuating them, does it?
Nor does it keep persons who need wheelchairs or any mobility device out of upper floors in multi-story buildings, off theme park rides, or many other places where evacuation - or merely routine exiting - becomes problematic if the elevator is unavailable for any reason. Why would a passenger car be held to a different standard?
Further, it is not only persons in wheelchairs who would benefit from an elevator - including in the sleepers with narrow aisles. Many people may have difficulty with stairs but do not use a wheelchair; They may have a cane or rollator or other device, or still be fully ambulatory, but just can't do the stairs. The elevator would be useful to many passengers even if wheelchairs can't navigate the upper level (though that begs the question: Could a custom wheelchair or transport chair be designed to fit the aisles and vestibules of a Superliner?) since once someone us up there, they have access to the diner and lounge with everyone else and aren't confined to the lower level of one car.