This is a tough one. It would seem that your management team is looking for a tangible value on something that is intangible!

I would take the approach that your proposed change will increase productivity. I can only guess that there is an application or report that takes longer to run with all of these useless indexes in place. If removing these indexes will help to improve performance for the end user enlist their assistance in building a proof of concept.

If you have a Development Region to play in make your recommend changes and then have a business user run their processes against the modified database and measure the difference.

The bigger problem it would seem you are facing is that management does not see the value in clearing out the "junk drawer". If you're not using it then get rid of it, thus you avoid becoming a digital pack rat 😀 .