Lots of things are dangerous Jim. Some things are more dangerous than others. When we have to choose between evils we need to strike the safest balance.
I live near the A34 and while I don't use it a lot I did worry about Mrs VB who had to drive on it most days for 25 years. At quieter times it was fine. When it couldn't cope it quickly (no pun intended) became a deathtrap. There isn't enough capacity https://hansard.parliament.uk/commons/2016-10-26/debates/F0B582CB-C642-4090-95F5-7B9972350634/A34Safety. It really needs at least another lane, but that's not going to happen. So the next best thing is to make sure it's used efficiently - no sudden braking or lane changes, no-one taking their eye off the ball, no-one wanting to drive much slower than everyone else or take up more than their share of the tarmac (and on a detail I'd ban mobile cranes completely for the hours between 05:00 and 22:00).
I don't think in the whole 25 years Mrs VB ever saw a dog or a deer (she did see a horse once that had escaped from an overturned horsebox).
I'm absolutely with you on reaction times. Perhaps they should be part of the driving test. And of the doctor's assessment of older drivers.
Where did the thing about overtaking on the left come from ? I don't approve of overtaking on the left and I try my best not to do it.That said, on a three-lane carriageway in heavy traffic it's not unusual for those in the outside lane to have to brake hard, sometimes to a standstill. I think if I was making more steady progress in the middle lane it would be more dangerous for me to stamp on my brakes to avoid passing even one vehicle in the outside lane than it would for me to ease off gently, passing a few if necessary.