I'm still hoping the customer service rep was wrong about Highway Assist not being included. The original press release Lucid put out a long time ago suggested it would be.
Lucid really needs to make this official with an actual statement somewhere. No more hearsay from random reps.
Meanwhile, on the question of resale value: I think it will certainly make a difference. But will you get the full $9k back? That's debatable. $9k minus however many years of enjoying the feature before you sell? Sure.
The fact that it cannot be retrofitted (as of today) might be worth more than the feature itself. Cars without it might not be attractive 3-5 years from now. So while you may not get $9k back, you may find a buyer that much quicker.
As someone who spent $6k on FSD for my Tesla, I can say with certainty that was not worth it to me. The features just never improved enough to be useable. From what I hear of Highway Assist, though, it already seems to be working fairly well. And I would pay the $9k just to have more cameras and the 360 view myself. Nuts, maybe. But that's my main reason for getting DDPro. All the autonomous stuff I consider to be a parlor trick in the short term.