This Helmet thing still has me a bit mystified. Just read and article that quotes $400,000 US a unit. Each is custom fitted to the individual pilots head for comfort and calibration.
The Helmet comes with the price of the aircraft. Well, the first one does. So there's 4000 Helmets. If you leave 2000 aircraft on the ground, and give 2000 pilots a spare, at least that save an aircraft being potentially grounded thanks to a Helmet malfunction at least.
And then there's training replacement pilots, so a course of 10 studes costs $4M straight up. You wouldn't want to flunk the course on your first week!!
It just seems to me like the weakest link, especially without an external HUD. Break a part of the aircraft and you hopefully have spares on site.