I don't really understand this comparison. Is it just that the iPhone X was a major redesign? But it was the flagship, and especially 7 years ago, people were constantly hungry for the latest greatest iPhone. Of course a flagship redesign was going to sell out! Only nerds argued about Face ID.
People are much more calm about phones now. And the Air isn't the flagship, it lacks some bells and whistles, but maybe most importantly the thinness makes people very skeptical about its battery life. That's a real issue for a lot of people. At $1000, the only thing really going for the Air is something that really can only be appreciated in person. This is something repeated by pretty much everyone who has handled the Air so far that I've seen--that the thinness and lightness is surprising. Given all these things, it's totally unsurprising if the Air didn't sell out in preorder, especially considering not many other models seemed to have sold out either.
No one actually knows if it's going to be a blockbuster--I certainly have no idea--but I think if it's going to be any kind of a success, it will be a slow ramp up as people handle it for themselves. Or it may fall on its face, who knows.
Also, I find it so odd that people want to label a new product a bust so quickly. What do people get out of that?