But isn't it also cruel to bench someone of Ohtani's caliber just because of a temporary slump?

‌The reason Roberts is considered a great manager is
Since Ohtani's condition was terrible, he didn't use him as the leadoff hitter in the playoffs and instead placed him in the 4-6 slots, taking all the measures he could.
He took a massive amount of criticism, but in the elimination game, he ultimately proved his trust in the players through the three-run homer (even though they were swept).
Honestly, if the players who played terribly are the ones getting all the hate, I don't think there's anything Roberts failed to do.
In modern baseball, just managing to repeat a championship among 30 teams is already a massive achievement.