Northbeach
Some valid & interesting points.
An aircraft manufacturer may come up against opposition from the certifying authority if it attempted to have an on/off switch for something that is certified as the PFD, however as I don't know if this approach has ever been tried, it is speculation from my side.
Another point with the HUD is that it is actually taking the pilots view away from the optimum primary field of view, when taking into account an instrument panel scan as a whole, with minimum head/eye movement.