Is rebuilding it into one modern platform a stupid idea? And would a VC fund it?
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"
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?
What does it do?
What is the motivation behind the rewrite?
Airlines are always facing issues with ticketing and mismanagement of assigning air crews and pilots.. introducing a modern ML accelerated flow to determine how to balance ticketing with crews for flights, taking into account historical numbers of passengers for that time/route could have an enormous impact on airline efficiency.
Maybe if you could attack just a part of the problem, extract it, then you might have something?