Can you explain how you have come to this conclusion? I assume you have test flown a PA28 with more than 2mm of ice adhering to the airframe.
I've certainly flown with at least 1mm of rime ice on the leading edge. Didn't fall out of the sky, and as soon as we descended it melted right away.
The whole point is that it is a complete unknown, noone knows how much you can get away with until it's too much and there's a serious problem.
Obviously, if it had been much more than 2mm, and we had intentionally stalled, things could of been more lively, but this was a simple IMC lesson, and picked up a small bit of ice shortly before commencing the approach, so I saw no big issue in continuing as normal knowing warmer air was only a few hunderd feet below.