In these debates, everybody who wants to look good will always take the position of an absolutely clean airframe.
It is a bit like arguing against CRB checks for PE teachers
But, as I said, there is a world of difference between muck on the wings etc, and muck on the hull. The latter is just a bit of friction. People fly with planes covered in dead flies... The former is what is going to get you because it messes up the lift.
And different planes have different tolerances.
I don't fly a PA28 (nowadays) but a few mm as absolute maximum sounds reasonable.
This is structural ice. Carb ice is a completely different issue.