The issue isn't the coffee shops. It's people. After college, it becomes socially unacceptable to initiate conversation with a person you don't know. People are not only stratified vertically by socioeconomic status, but horizontally by industry- and subculture-specific senses of superiority (academics vs. "techies" vs. bankers).
A coffee house would have to work at it to recreate the old dynamic: hold board game tournaments, have common viewing of intelligent television (when it's on, which is rarely). But that would conflict with the business goal (except in college towns) of getting people in and out reasonably quickly so you can afford rent on the space. The truth is that most people don't want to meet new people. They want to work on their laptops and talk to people they already know.
"After college, it becomes socially unacceptable to initiate conversation with a person you don't know."
I don't think this is actually true. My girlfriend will chat up basically anyone she meets - she'll talk to the guy making her burrito in Spanish, crack jokes with the checkout cashier, introduce my slam-poet friend to her CFO's teenage daughter who wants to get involved in the arts scene, cold-call alumni from her school asking for career advice, respond to cold-calls from people asking for career advice, and randomly strike up a conversation with a stranger on a street corner. Occasionally it leads to some awkwardness, like on Halloween when she asked the guy behind us if he was Sheldon Cooper for Halloween (he wasn't wearing a costume). But most people are very glad to be treated like a human being and talked to directly.
I think that what happens is there's a selection effect at work. People don't try to maintain a conversation with other people who don't want to be talked to - that's rude. And so if you believe that most people don't want to be talked to, you'll give off "I don't intend to talk" signals, which will ensure that most people don't try to talk to you.
My last girlfriend was like that. Two things: 1) she was very attractive; 2) she was outrageously gregarious.
I can accomplish a fractional amount of her socialization if I happen to be feeling supremely gregarious (not very often), but for the more averagely social human being, it can be challenging to initiate conversation with people you don't know. I do think the built environments of the adult, professional world are less conducive for this.
On this point though, I always marvel at the park picnic scene: often you can find very large groups of ethnic or seemingly blue collar people holding court at the picnic tables at parks. My own personal demographic isn't as frequently represented in that scene. Go ahead, guess.
My wife is like this. We have an agreement where she goes to more of the children's birthday parties. She loves to socialize with strangers, me not so much.
A coffee house would have to work at it to recreate the old dynamic: hold board game tournaments, have common viewing of intelligent television (when it's on, which is rarely). But that would conflict with the business goal (except in college towns) of getting people in and out reasonably quickly so you can afford rent on the space. The truth is that most people don't want to meet new people. They want to work on their laptops and talk to people they already know.