Yes like any tool, LLMs can be used poorly. But they do enable large-scale rewrites or poor-quality code because they are capable of understanding it and enumerating its behaviors in a matter of minutes or hours. It's the kind of understanding that would take a human months or years to develop.

Add to that their ability to basically set up what is essentially a perfect testing environment when asked, and you've got a feedback loop that lets you just step back while the agent cranks out a new implementation in a memory-safe language with a full test suite and bug-for-bug compatibility. I'm not kidding or exaggerating. This stuff is possible now, people just need to look past their anxieties about being replaced.