Hi there,

Yep, you could certainly substitute a simple "not X but Y" here. The "rather" is always optional after the "but," and its function is really just to emphasize the distinction between the X and Y. For instance, I'd probably say, "He's not a child, but a teenager," since I don't care too much about making a very big point of the distinction. But I'd probably say, "This was not an unfortunate accident, but rather a malicious, premeditated crime," because I really want to emphasize the distinction between what it was and what it was not. That's subjective, though, and grammatically you're safe either way.

Cheers!