You need to ask other questions too. How often are toaster-induced fatalities triggered by end-user modifications? How often are car-induced fatalities triggered by end-user modifications? How much could these rates go up if large-scale modifications were permitted?
I think you'll find that most toaster-induced fatalities are fires and electrical-system failures (particularly in the US with your literally terrifying electrical regulations, plugs that can shoot sparks when you unplug devices and all that): modifications to toasters are rare and generally the worst you'll do is make it not work. I think you'll find that most car-induced fatalities are caused by human error and/or the car functioning as designed, or by failure of manufacturer-provided systems. Car hardware is modded all the time: do those mods increase the fatality rate perceptibly?
I have no idea if this is true: I don't drive. I know that *some* mods, e.g. bull bars, *do* increase the fatality rate, but in the US, land of the SUV, bull bars are probably provided by the manufacturer and don't count as end-user mods. Heck, I wouldn't be *too* surprised to find the manufacturers providing sharp spikes on the front to get inconvenient pedestrians out of the way, and hood-mounted cannon to rapidly clear those unfortunate traffic jams. :P
I note that it is perfectly legal to modify your car to do all sorts of things, but that in the UK at least you have to regularly pass a test to make sure the thing is roadworthy before you can drive it anywhere but on a private road: so mods that made the car notably more likely to roll over onto pedestrians would probably be detected. It seems unlikely that mods to in-car entertainment systems would be anywhere near that dangerous -- but it is also true that your average garage owners cannot possibly diagnose faults in the software the way they can diagnose problematic hardware mods, and there's not much chance of government regulation keeping up with it either (at least in the UK, this is regulation enabled by legislation, not legislation itself: the MOT is routinely revised without Parliamentary involvement, but revisions to cover software mods seem hard to implement).
So I am of two minds -- and literally an outsider in this, since I'm about as likely to learn to drive as I am to learn to swim in molten rock.