Shigaraki, despite being a mass murderer, had a genuinely impactful backstory that made a lot of people sympathize with him. He was abused, ignored by society, and ultimately taken in and shaped by the worst possible person. He didn’t start out evil he was basically molded into it. Dottore, on the other hand, was already messed up from his student days (if you played the last sumeru event you would know). He didn’t need someone to push him into becoming a monster he chose that path himself. Everything he does is for his own benefits, with zero regard for others. And unlike Shigaraki, he hasn’t been given a backstory that really tries to justify or soften what he is (at least for now). That’s why Shigaraki just doesn’t work as a replacement for Dottore they’re built on completely different types of villain writing.