I agree with that, but let me inject a little bit of reality.
How long has the PA28 been out? A few decades. Countless tens of thousands are (or have been) flying.
I simply do not buy that 2mm is going to bring one down. I am not a 20,000hr pilot (and anyway most of those do it in a big jet) but I have seen enough IFR, and since nearly all formal IFR I do is high altitude (Eurocontrol flights) I have seen plenty of icing. Summer or winter,
some icing on almost every flight, FL140+.
Most of it is just light, but 1-2mm is really easy to pick up. You can get it in seconds. I wouldn't bat an eyelid at 2mm; it is barely visible as a bit of frost in between the rivets on the leading edge, and it doesn't show up on the speedo at all. And the shape of my wings is not much different from a PA28.
If you fly a plane which will plummet at 2mm (and I am sure there are some laminar designs that will do that) you better forget
all flying in IMC below 0C unless you have full TKS (£30k or so - more than most PA28s are worth) and you use it - ~ £200 per hour is what it costs. Or rubber boots, and they better be working (many aren't) but few modern and potentially ice-sensitive designs use boots.
My guess is that 2mm of rime is going to increase PA28 Vs by a few kt. So you won't be doing a max performance landing into Deanland
I am also pretty sure there is plenty of urban knowledge on this, but they are not going to be posting it here because most are renters and are going to get a bad name
What you can't do is just sit there and let it build up and up and up, with no way out (which usually means a descent into warmer air). If in IMC below 0C you have to watch it like a hawk and be considering your options. But that applies to any non-deiced plane. One cannot embark on a flight in potentially freezing IMC, for 1-2hrs, with no way out.
If you have an IR, then you would file for FL100+ and climb up through the thin layer discussed and sit there in sunshine. With the IMCR you are basically a "VFR" flight as far as ATC are concerned (declaring yourself "IFR" does not get you anything) and you get pushed down below CAS, and that is the one area where an IR is useful in the UK over an IMCR. With the IMCR, winter flying is mostly going to be below cloud unless one can be sure one can climb above it without going into CAS.