Is rebuilding an entire legacy software system stupid?

I'm sure theres stories of "we re-wrote everything in $(modern language) on time, in budget, and now everything works so much better"... I've never heard one tho.

If you want someone else to fund such a thing, it's probably gonna need to be something that has immediate, slam dunk sales prospects at then end... like "we're gonna re-write the post office systems and we know we can sell it because the Postmaster General is my father"

>still (inefficiently) in use today.

What's inefficient about it? Computers are millions of times faster than they were, you're not worried about CPU cycles are you?

Is there a business need that the software can't meet? Or, is it just the general hassle of dealing with legacy code?

It's difficult to say anything without context.

What does it do?

What is the motivation behind the rewrite?

If you're asking yourself if it's a stupid idea, it sounds like you already know the answer...

Maybe if you could attack just a part of the problem, extract it, then you might have something?