The Pilot ought to have a foldable HUD as a back up for the Helmet display.

Small electronics are so prone to reliability issues, and if the aircraft has battle damage where the Helmet is powered and he does not have a backup source of flight data hes in deep trouble.

Modern tactical rifles, have red dot low power optical sights that make accurate fire that much easier, but because optics can break, need batteries, or they might not be zeroed in you still need back up iron sights.

My foldable/ popup HUD would function the same way as the back up iron sights. Might not need it much but nice to have when you need do it.