Well I guess if we're talking "life or death" here, then of course the percentage would be higher. That's kind of a silly layer to add to the discussion, because it has no basis in reality. When is running a sub-56 400 EVER going to be a life and death scenario? (Yeah yeah, if you're being chased by someone/something that intends to kill you, maybe. But that's not the scenario being proposed here.)
In reality, there is no consistent, reliable way to control for motivation when asking the question, "What percentage of 20 year-old males could run 55.xx in the 400m if trained?"
Also, if everyone were motivated by a life or death ultimatum, and injuries never occurred, then the standards for what constitutes a fast 400m time would completely change. It would change the standards for all distances. Would the mile WR still be 3:43 if all young males were motivated by life or death and no one ever got injured? Probably not.
Let's be a little more reasonable here. Take a random sample of 20 year old males from the US (let's say 1000). Give them 2 years to train and a reasonable financial incentive to break 56 seconds. Give them all the same coach; someone who is at least somewhat experienced and basically knows what they're talking about. Yes, he will have his hands full. He doesn't have to see them all in person every day though. Split them into groups of randomly assigned training partners. Assign a doctor and a small team of trainers that can help with rehab if injuries occur. And so on.....
My prediction: somewhere around 10-15% are successful. When it comes to the 400m, raw speed is the main factor. There is some strength and conditioning necessary, as well as coaching on mechanics, strategy, build, etc. But basic speed is the main limiter here. WR mile pace is about 13.9 seconds per 100m. So all participants will need to be capable of running an all out 100 faster than 13.9, as a bare minimum requirement for potential success.