I don't think it's ageist to be worried about older drivers although I think in most cases 75 is too young to worry. From 85 it's a different story. If you see someone driving far too slowly, 99 times out of 100, they will be elderly. If you see simple bad driving, that's usually a mum in a car that's far too big for her. But both of those are generally low speed accidents. It doesn't make it right though.
I was interested in the comment further up about Parkinsons. My father has Parkinsons and finally gave up his car at 90. That was two years ago. You're right, he saves money by not running a car. But will he get a taxi? Nope. Too tight-fisted (and he's really not hard up by the way). For a while he got a lift to his local supermarket with a guy who lives in the same sheltered housing complex, but then there was something wrong with his car and he decided to give it up. So I said I'd organise a Sainsburys shop each week for him. But he wouldn't do that after a while because he'd never order enough for it to be free delivery. You'd have thought £6 for the luxury of having it delivered to your kitchen and no hassle would be ok, but no. So now he phones his local Coop and they deliver for free but he doesn't like their products as much as Sainsburys, sigh. I digress slightly but just wanted to make the point that an elderly person can effectively housebound themselves, even though they don't need to.
My mum has a friend who is well under 90 and he didn't need to take a test and never has. Does it show? Not really. He did get caught doing more than 20 in a 20 limit but frankly anyone could do that, it's nothing to do with age. He went on a speed awareness course.
I did my test in 1990 so I didn't have to do reverse parallel parking, but I can do it. In fact I can reverse much better than my DH, who did have to do it. Ultimately you learn this stuff after the test.
As for satnav, I never use it. Looking up the route on Google Maps before I go and keeping a road atlas with me works fine and I don't get sent down narrow lanes under low bridges. That's another category of dangerous driver actually. The ones who realise they've gone the wrong way and slam their brakes on, or crawl along. If you have gone the wrong way, that's the worst that can happen? You need to turn round somewhere. Not so easy on a motorway but still not the end of the world and anywhere else you'll find somewhere within a mile or so. But don't crawl along paying no attention to the road.
I don't think we need retesting but a regular refresher would not go amiss. And maybe everyone should target doing the advanced driving test at some point.