But I do agree that most would be extremely bored and feel useless if they retired at 40.

My dad retired at fifty and just started helping people for free. He helped build houses, taught math and physics at a private school, built another house for himself, etc. and he has done fine.

Takeaway: if you retire young and are still involved in meaningful work/service you might be fine. If you retire and just hike and play golf you’re probably going to struggle.