Actually, the HMDS would have been easier and cheaper if it hadn't replaced the HUD.
If you have both, you don't have to make the HMDS tracking system gunsight-accurate, and you can easily put a high-rez image on the HUD for tasks such as landing, where you want lots of clear texture cues. So you don't need a special new-tech camera in the HMDS, or at least not as badly.
And - well, you have both, in case one breaks.
The reason that there's no HUD (as I understand it) is that they really wanted the big wide-screen display. In 1995-2000 this was not compatible with a HUD because the optical chain got in the way. With new waveguide combiners, this is no longer a problem.
