Yeah. I live in a nice building in modern-day Manhattan–not as nice as Don’s, but still a nice place with elevators and a doorman–and our stairs look more or less like Don’s. Nobody uses them for much of anything at all–the only reason I’ve ever been in them is the hurricane, plus our laundry is on the 2nd floor, so it makes sense to just walk down to the lobby if I’ve just dropped off a load of clothes and am going to step out. The only real difference between mine and Don’s is that Don’s is more spacious, has doors into individual apartments, and has a different paint job.
Another presumed difference is that in Don’s day, everyone in the building had servants who used the back hallways all the time, maybe exclusively.
Not relevant here–this is a pre-war building, built in something like the late 20s. 
It’s just not quite that high-end.
That snobbiness still exists in some quarters. This article from last week’s New York Times mentions that some co-op buildings in Manhattan require everyone but the residents and guests to use the service elevators.
Clearly not the best ep but my main problem with it was what I perceived to be the over-the-top drug induced antics. This in a show that tries so hard to get everything about the 60’s right.
So am I wrong about the excesses?