If running chest-out was penalty-free, Usain Bolt would be doing that for all 100 meters, not just the last 5 or so.

No, you missed the most important reason for sliding/diving, from the viewpoint of a base-runner: on a non-force play where the fielder must tag the runner to record the out, the slide/dive makes the runner as small of a target as possible, and any loss of speed from friction is made up for by the extra time needed by…