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.
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.