I don't want the 70 year old with less than perfect reflexes on the road. If you can't operate a vehicle with proper motor skills (haha), then get off the road, plain and simple. We recently took my grandfather's keys away because of this.
The whole idea of "the" speed limit is that the roads are in perfect condition. It would be nonsensical to lower the limit for every single day because it snows 50 days of the year. Drive to the conditions, we all do (or should do) this in Ottawa every year, especially on the Queensway.
Basically, I disagree with your entire post.
But that's okay.
Re: learning to drive and the implied intimidation of driving fast, I really think we should have the big N or L on the back of the car for new drivers like they have in Europe. I gave a lot more breadth and had a lot more patience for cars with L on them when I drove over there.