It's not about guaranteeing they score, it's about setting yourself up with the best possible players TO score. If they went to the 10th penalty kick and they HAD to use Komano and he missed it the way he did, then it was his responsibility.
The coach needs to put players in the best possible scenario for themselves and the team. You do not send out a player who has NEVER scored internationally in the first round of PK's to determine if you get eliminated or go through. It was stupid.
They HAVE put their best players to score, all the teams practice penalties and have a good idea which players are mentally the most stable to take those shots. Maybe he does that often for his club, or was good in trainings, who knows.
It's not fair to criticize after-the-fact that this is the reason he missed, because there are multiple examples where top players/scorers did the same, e.g. Maradona in 1990 missed a penalty in 1/4 final being the best player in the world at the time, R. Baggio in 1994 finals (and he was taking all penalties in regular play), Beckham who scored many goals from free kicks missed the complete goal from a penalty on one occasion etc. Saying that it's unacceptable to miss the goal on that level is a misunderstanding of the kind of stress these guys are under with half the world watching...
And all said and done, penalties are mostly luck. The Japanese GK went to the right side in the first 2 shots, but they were too accurate. In the next 3, he picked the wrong side, and in almost all of them, esp. third and fourth, if he had guessed the right side, he would have made the save, the penalties were quite meekly executed.