فَإِنْ لَمْ تَفْعَلُوا وَلَنْ تَفْعَلُوا
If you do not—and you never will—do so.
The rhetorical logic is an argument: a challenge is issued, the audience fails despite opposition, and the failure highlights the exceptional character of the challenged discourse. Incapacity is the intended demonstration.
The term mithl (“comparable”) needs an appropriate standard. Superficial resemblance in one feature is not necessarily the same as meeting the challenge in its relevant literary and communicative qualities.