I actually don't think it's true at all. It's easy to get work done, but the key part is that it's easier to stay at the office longer - eat lunch in the office, take a break to play ping pong for half an hour, have a beer with your coworkers towards the end of the day... it all results in you spending more and more time in the office, and leaving later in the day. So even if you aren't as efficient as you would be otherwise, the company makes up for it.
I suppose it differs based on your goals at work. As I get older, I seek much less camaraderie/company in the place of work. Ideally, I'd be mostly left to my tasks, complete them in 6-7 hours, then leave so I can pursue my non-work life (the one I value most). Most employers do not want to hear this.
Likewise, I do most of my work between 7 and 10.. after that, the loud crowd arrives for their 'morning' and start discussing loudly about irrelevant things, and the concentration is gone...
Kids come along, you become more choosy about the people that you socialise with, free time disappears in an instant.
You still enjoy your job, but you can no longer justify spending more than 9-5 doing it, so what becomes more important then is to actually be productive during the 9-5
This means quiet, efficient "grown-up" offices.
Except a number of things that are associated with lack of productivity in startup culture have actually been shown to increase productivity, like open floor plans.
open-plan offices are not new.... I've been working in offices for the last 30 years across ~15 different companies (I'm a contractor) and I've never worked in a non-open-plan office.
