Business logic is usually the most substantial part of legacy systems in my experience, so I imagine so.

Not to be too negative but a lot of modern software complexity is a prison of our own making, that we had time to build because our programs are actually pretty boring CRUD apps with little complex business logic.

I can only assume there's a ton of domain knowledge accrued over those years and beyond baked into the legacy code, that an LLM can just scoop up in a minute.

Not the poster you replied to but I’m sure it did. But still manual rewrite under the same constraints would be much less feasible.

Yep, coders do more than just code.