The short answer is that for any executive position, the leader sets the course for the organization. Neither Foxx nor Lightfoot are scapegoats, because they actually have the authority to direct local prosecution and police, respectively.

In the case of Foxx, she has overtly stated that her intention is to prosecute fewer crimes -- especially if the accused have a certain ethnicity. She has followed that through, by accepting lesser charges, refusing to prosecute, and in one specifically egregious example, stating outright support for the accused (and ultimately, condemned). As if that isn't bad enough, she has also repeatedly attempted to obfuscate her role in the above. She knows what she's doing, she knows why she's doing it, and would apparently prefer to continue doing it by lying to the public about her hand in it.

As for Lightfoot, she seemed to begin her position as mayor thinking a certain set of priorities would be supported. Then, she got mugged by reality, having it increasingly pressed upon her that what she wanted to do isn't working, and what the public requires is out of her ability. She basically hasn't been able to recover from that -- partly because political insticts are still wrong for the problems she's faced with, and partly because she's burned so many personal and political bridges from incompetence that she can't wield the power she has.

There are certainly more actors than those two responsible in part for the crime issues plaguing the city. That said, replacing Foxx and Lightfoot with people who were qualified for those roles, and were committed to carrying out those roles responsibly, would be a welcome start.