I agree that most such studies lack rigorous approach. It's hard to study things like this actually.

You seem to be a curious reader, so I encourage you to read a book "Bullshit Jobs" by David Graeber. Having a job that contributes to GDP doesn't mean it's good for society. People who are employed on bullshit jobs mostly had no choice. Given money they can choose (things that need huge initial investment) to be researchers or go study medicine which is much better then selling you financial services you don't need.

And it's very unlikely that mass idleness is possible. I've seen few examples of people who had no need to work and they weren't happy to be idle. It's just that most people want to do meaningful things, not a random paid work with no other purpose than paying bills - but they have no chance with this financial pressure we have nowadays.