Don't the qualifying rounds determine whether you get to compete or not? So if someone intentionally slowed down, even a fraction of a second, there's a chance they wouldn't get to compete at all.
Yeah, but it would create weird incentives where e.g. if two people are way ahead of the pack, one of them wants to finish after the other. So do they just wait until the rest of the field catches up and then try to trick the other guy into hitting the wall first?
Better to just avoid it by keeping incentives aligned with winning your qualifying heat.
When the US 4x100m swim relay beat France about three olympics ago (the one where Phelps won all those medals), France was comprehensively faster man-for-man aside from Phelps in that relay.
The US won because Jason Lezak, the anchor, swam in the wake of the final French swimmer (a draft basically) and was able to save energy and pass him.
If the US and French had been separated by a lane, the US would have lost.
If you are slightly faster (and especially if you go out faster) than your #2 rival, you want to have a lap between you to deny them the draft.
