I live in Cornwall, so don't actually drive on motorways that much, but do drive on a lot of A roads where the speed limit is 70. And yes, you do have to be aware of things like farm vehichles driving on these roads- but that is generally fine as you can usually spot them at a distance and prepare to take evasive action.
The slightly lost tourist on the other hand who is doing 40-50 or less- harder to spot and prepare for, so a greater hazard.
There is a reason you can fail or get minors on your driving test for driving too slowly (I think they call it failing to make apropriate progress?), and that is because you are a hazard. I am pretty sure that you would fail if you did 50 in a 70 zone where it was safe to do 70. If possible, you are expected to drive at 65mph or higher.
I do think there are lots of older people who passed their tests when there were a lot less cars on the road, and things like this did not matter quite as much. This is why I think people should be subject to retesting as they get older. My dad has turned into one of those people who drives at 40-50 miles an hour everywhere. He thinks I am an unsafe driver because I drive up to 70 when it is ok to do so. On the other hand, he is the one with points from doing 40 in a 30 zone.
Yes, speed can kill, but this generally applies to areas of slower road, where you might encounter unexpected hazards. For example it is not usually safe to be driving at the speed limit on country lanes, or speeding in a residential area where a child might run out into the road.