True, but neither was a forward facing radar, multiple new sensors and all the associated wiring when that came along in the S. Owners were receiving cars and had no idea what all this "stuff" was because the hardware pre-dated the announcement and software release. If someone at Tesla (i.e. Elon) thinks HUD is a good idea, then you can bet at some point it will show up.
BTW, this has been a very frustrating thing for S owners. Tesla doesn't iterate by model year, they just start adding stuff as soon as it's ready. Many buyers have "missed out" on these upgrades by literally days. Sure, they got what they ordered and paid for, but it's gotta sting a bit when, for example, AP hardware is magically released the day after you take delivery.