Technical debt and its costs are extremely hard to quantify and saying things like "with 3-6 weeks of engineering time we can cut failures in half" is just inventing numbers to get what you want.
Admit it or not, the reason you want the technical debt addressed is primarily emotional. You are frustrated with how difficult it is to work, you fear for the future of the project. If you want to compel someone in charge to make changes, you need to make them feel things are bad as well.
This is to be used responsibly, though. If it turns out you've convinced someone into acting in a way that is against their interest or a big waste of time, there will probably be blowback later.