In both cases, one possible position is being asserted as THE correct position, even though another position might be correct. So they’re two different sides of the same coin. “Takes for granted” is the wording that describes that position being chosen (i.e., it’s an assumption), while “fails to consider” refers to the position being ignored.
For instance, if I try a Coke Zero and proclaim it to be the best diet drink ever, I’m “taking for granted” (i.e. assuming) that Pepsi does NOT have a product that’s better.
I’m “failing to consider” (ignoring) the possibility that Pepsi DOES have a product that’s better.