Are you considering jobs that are extraordinarily demanding? What if you're an ER Doctor? Or an Air Traffic Controller? Or someone getting started in their career in their early 20s, when most of us possess the unique combination of a lack of life experience that would prevent exploitation and ambition? For these jobs, I can easily sympathize with the idea that after a workday they're too tired to develop personally. Moreover, it's a manager's job to sap every ounce of productivity out of a person. Modern technology increasingly makes this possible. Even seemingly mundane jobs like working in a call center can be so orchestrated that using the bathroom makes them fall behind. And productivity has done nothing but rise for decades!

I also don't see how your final paragraph really refutes rather than just restates their opinion. Hobbies produce projects and business ventures all the time. Someone also has to find some way or another to socialize with the community. Volunteering is a great way to do that.

Jobs that you want to do vs jobs that you have to do are entirely different levels of motivating and result in completely different energy levels for people.

If I have to do a job I hate for the rest of my life I would eternally be low energy. If I could do the thing I loved every day, the thing I truly wanted to do, I would get up excited every day and would have high energy throughout.

Having more free time, yes people would get bored. But the resulting things that they work on would be things that invigorate them.

Fortunately, there is an immense variety of jobs to choose from.

"9-5" jobs in America are anything but. Since the popularity of smartphones people are reachable 24/7 and employers are taking full advantage.

Hourly employees have it even worse. When your schedule varies week to week and even on your "day off" your employer may be constantly reaching out trying to bully you into taking another shift, it's very hard to maintain regular non-work activities. Perhaps you have friends who work similar schedules to you, but good luck going to a sports team or club that meets Thursdays at 6 when you don't know if you'll be available then until 12 hours before if ever.

That's a 24% change which is well above the threshold of significance.

People sometimes underestimate how much work goes into a new business. The idea that you'll fill "spare time" with a business is laughable.

(I'm not thinking of a making-money-from-my-hobby side gig, but an actual business.)

You can do a side-hustle in spare time, but an actual business, one that pays salaries every month takes enormous effort.