Why did Sauron fear Aragons coronation?

[Galadriel55]

I think that Aragorn was referring to more general traps, against a Numenorian warrior brave enough to stick himself in Sauron's path. I doubt those traps were even aimed with the same person in mind, or for a greater purpose. They're just traps.

As for Sauron apprehending the heir of Isildur, I don't think he thought him dead as much as lost. I think he wasn't fooled by the destruction of Arnor into thinking that Isildur's line ended, but since the heir did not show a sign for so many generations he probably thought that the lineage was forgotten, or lost, or too mixed. He was still wary of a potential heir, or a Dunedain leader with enough lineage to be proclaimed King, but not as much as he would have been had he known the truth about Aragorn (which he became after he saw him in the Palantir).