Litz wire doesn't increase the durability as such, merely reduces skin effect in RF applications. Sennheiser used to sell steel cables for their professional headphones. Now those are durable. I ran over mine with my chair about 50 times a day and the cable was still going after 10 years.
Steel makes sense for extreme durability - I suppose you could use extremely thin steel wire for in-ears to make it flexible, though it might require a very special alloy to be thin and low resistance?
In my experience, litz wire is exceptionally flexible and suitable for getting shoved in a pocket frequently. Each wire can be quite thick due to the flexibility.
