I think time machines would be used to opposite effect if they existed. If something bad happens, you just rewind a few minutes and say "hey Jim, be careful on that ladder, eh?"
Depending on what you mean by "at least", the warning could still be a very effective safety mechanism, or we might realize there's no point in trying to be safe at all, or something else entirely. I think the implications need to be nailed down before a convincing argument can be made.
