Commercial real estate is about $3/SF (per month) in San Francisco (Class A) and $4/SF in Manhattan. Open plan offices save about 75-100 SF per employee (more, if you're looking to be sadistic and impractical, but at this point, the productivity loss is obvious.) So the employer is saving, at absolute most, $5000 per employee per year. It's not worth it.

I bring this up because I just really truly don't get the occasional open office boosterism here. My current theory is that most of it comes from people who are still in the honeymoon phase of their career (like, the first few years after college when everything about the workplace still has some novelty to it).

That's basically what it is. The open-plan layout is reminiscent of college and the all-nighters in the computer lab. But work is not college. A bad grade on an assignment is much more fixable than getting fired. The stakes are higher, and that's why a certain brand of professionalism (or, more bluntly, divorcing oneself from behaviors that are acceptable for 18- to 21-year-olds) is necessary.

I say this because it seems like many/most of the people I talk to in person who like the open office idea have either literally never had a private office, or are relatively fresh out of college and get a job at Google (or a startup, or wherever), and they're still so excited about everything that even the things that are hassles (like open office plans?) seem like they're brimming with novelty.

Well, private offices make nerf-gun battles harder. And you hear less juicy gossip. And you don't have an audience for self-important rants about Lena Dunham. It might get so "boring" (in the sense of a lack of distractions) that you might be tempted to actually work to pass the time.