I'm not the parent, but now you have a disagreement (it's not solved; solvedness is a continuum) not a fallacy.
I don't think it's in good faith to assume that because parent said "perfectly appropriate Sans Serif typefaces exist" that the implication is that solvedness is a binary; the implication could just has easily been that the problem is sufficiently solved so as to not matter, which is a disagreement about where to draw a line, not an abuse of logic.
I don't think it's in good faith to assume that because parent said "perfectly appropriate Sans Serif typefaces exist" that the implication is that solvedness is a binary; the implication could just has easily been that the problem is sufficiently solved so as to not matter, which is a disagreement about where to draw a line, not an abuse of logic.