I think it's very hard to make the argument that speed, in isolation, kills, especially on the motorway /freeway etc.
Speed plus will get you though. Be it speed plus inexperience, unfamiliarity with the road, or meeting a car that has no idea you exist
Cars are so much safer these days, but the level of inattentiveness when driving is spectacular - anyone here not been knocked off, nearly taken out, or know someone who has been taken out by a car changing lanes right in front of you? I was only going 40mph but I still didn't have the room to stop or dodge.
I ride most places locally at pretty much the speed I feel comfortable at, which on occasions is in agreement with local laws. I accept that may at some point cost me money (amazingly, only once in 20 years), but I don't feel it puts me more at risk.
I drive my car much more at the legal road speeds, one because I'm not generally driving just for enjoyment, and two, messing up on a car is rarely a problem that confines itself to just you and your vehicle.
I don't know what the right thing to do with speed limits is. I'd like to say that, as has been said above, people are generally not idiots and will drive to their ability and road conditions by default. However, this was said on a VFR forum not a GSXR forum, and we wouldn't be riding what we ride if we didn't all have some sensible (and middle age) in us. Not all do.
I guess ultimately I'm glad we have the restrictions we do. It makes it safe to be out at legal speeds and have reasonable confidence that the many people driving on autopilot aren't going to kill me, and leaves me with the choice to crack on at higher speed if I want, assuming I'm paying enough attention to not get busted. The idea of thousands of people driving home knackered at higher speeds because they can does not fill me with optimism!
Worth noting that where I live, the chance of getting up to legal speeds in a car is frequently low owing to bad traffic, and we also have horrific road surfaces with many potential bike mauling potholes.