Random turbulence that maims the flight crew just wouldn't be practical as a "thing that just happens" on regular longhaul flights.

I never said it happens often but it absolutely does happen. Here was a particularly spectacular example that happened to folks a few years back on their way to Australia (and note, if you want more examples, the article lists a couple of other past incidents that also resulted in crew and passenger injuries):

https://apnews.com/article/49db2788d04d4e11bcbb1a63dbae4199

Passengers on a flight from Canada to Australia said they had no warning about turbulence that suddenly slammed people into the ceiling of the plane and injured more than three dozen — a phenomenon that experts say can be nearly impossible for pilots to see coming.

One passenger on that flight noted:

“The plane just dropped,” passenger Stephanie Beam said. “When we hit turbulence, I woke up and looked over to make sure my kids were buckled. The next thing I knew there’s just literally bodies on the ceiling of the plane.”

So again, I cannot emphasize this enough: wear your damn seatbelts, people.