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"