In a nutshell, a large portion of the danger in F1 at the time in the mid-60s to early 70s was that the progression of technology that enabled racing cars to go faster (Wings, spoilers, monocoques, stressed engines etc) was going so much faster that the track owners didínt & wouldnít keep up with and became antiquated. Circuits like Spa (of old) and Nordschleife were part of a bygone age and grand as they were/are, they had to move with the times or be left to history.
The cars (or their designers) were just as much of the problem with safety towards the driver coming last in the list of priorities of the pursuit of speed. I canít remember who in the program said it, but once the technology that makes a car quicker is found, it canít be unlearnt.
Regarding seatbelts, looking at it, I can fully understand the drivers not wanting to wear them, I wouldnít want to be strapped into a molotov cocktail on wheels either.