>For example, the vast majority of people choose to retire once they reach the age where they are able to collect enough from their pension that they no longer need to work in order to get by

Part of the problem is that the current system doesn't provide a great way to taper off, at least not by default. I suspect there would be a lot more people who'd keep working if it was simple to get a comparable job at 30 hours per week 25 weeks out of the year. But for those who are traditionally employed instead of contracting, the choice is often between full time or nothing.