Men don’t pass on their mitochondrial DNA, so the rarity or otherwise of the iceman’s mitochondrial DNA has no bearing on the question of his possible descendants.
The “line” that is averred to be extinct includes his mother’s mother’s mother’s ... mother’s mother ) whoever is considered to be the “head of the line”, ) her daughters, daughters’ daughters , daughters’ daughters’ daughters, and so on to the present, and only the living sons of any of these “nth daughters.”
In those days, in those isolated, sparsely populated areas, his kids might have shared his maternal DNA anyway. (which is all beside the point)