I see. Your'e saying: since speed limits aren't perfect in all cases, so lets not have them at all.
I do believe in increasing personal responsibility of people and having less of a nanny state, so I do tend towards that answer, but it's really not that black and white. On highways and places where pedestrians and bicyclists are not allowed - yes, no speed limit makes a lot of sense (and is available in Germany for example)
In cities and suburbs where kids can run into the street in unexpected places - more thought is needed what is the best solution.
I've actually gotten a speeding ticket because I was going 35km/h in a "construction" zone with a temporary 30km/h limit on a freaking Sunday with nobody around. That is also clearly ridiculous.
Also, the notion that there is some universal speed limit that makes driving safe on a particular road is silly.
Two scenarios with speed limit of x: 1) sunny day, brand new porsche, new tyres. I'm driving at a sped of x+5 2) windy, snowy day, nissan sunny from the 80s with almost worn out tyres. I'm driving at x-5
Pick the safer one. I doubt anyone would think option 2 is safer, yet 1 is illegal and 2 is perfectly fine.