Dadge, what you are saying is true. The race should probably just finish at the finish line.
But since we have this "slow down lap & park at pitlane after race finish" rule we must have a penalty because anyone who doesn't use the extra fuel for the last lap (~3,7% for the BF1 at KY2) will have an advantage for the whole race cause he will have a slightly lighter car then others for the whole race.
So we must either keep the 30 secs penalty as it is, or simply scrap the "slow down lap" idea.