The tragedy of the Shimura Family is that Tenko's family did stand up for him but he never saw it, and they would have been able to heal without All for One's interference.
Even if Kotaro couldn't get his act together, Tenko and Hana would have been okay. Of the enabler parents introduced in BNHA, namely Nao, Rei, and Tomie, Nao stands out as the one who was taking concrete steps to protect her children from abuse.
Perhaps she was a stronger or more well balanced person, but to be fair to Rei at least, Nao had more leverage over Kotaro than Rei had over Enji. Kotaro was an orphan. Nao had a good relationship with her parents. Nao provided Kotaro with the support of an extended family, and she had the power to take that away if he harmed their children. Rei didn't have the support of the Himuras and they sold her into what they knew was a transactional situation.
Nao was easily controlling the conversation here. She wasn't letting him bat aside the severity of the fact that he resorted to violence. She gave him an ultimatum of what was at stake if he crossed this line. Kotaro's rules would no longer be in play. Whether she meant they would all ignore his insistence not to speak of heroes or they would all leave is up to interpretation. She brought her parents both as a demonstration of the unity of the family against his actions and to act as witnesses in case he continued to fly off the handle. Nao clearly had a good head on her shoulders.
That's the core tragedy of it all. Without that final push of giving Tenko Decay, he would have been freed from that suffocating house without the grief either from Kotaro's reform or Nao taking him out of it. He would have learned that his family did support him even if they were a little late.
