Over the last decade or so I've gotten the chance to use a number of antenna /accessories made of uwave RADOME plastic from a few different vendors and in all cases the plastic was cracked, crumbling, taped together, etc. It seems like a systematic problem.
I bet there are a bunch of polymer chemists who know exactly what's going on.
I have a feeling that it's a combination of two things.
1) The plastics might need to be made out of a very specific material which might not be very stable in the long term.
2) RADARs get freaking bloody hot and even a few % of absorption might be enough to essentially make them into a microwave oven.
I'm guessing some combination of the material, pure heat and radiation might be the cause of these plastics breaking down but I'm not a material engineer however this is something that I've also encountered.
I have had experience with SAR pods and their RADOMEs have had to be constantly replaced due to degradation as well.
In fact in spot/strip mode it wouldn't be uncommon to see actual discoloration of the RADOME after a few flights.
Most of the crumbling RADOME material I've seen has been on receive-only antennae, because that's what I deal with regularly, so I don't think it's purely a power thing.
My working hypothesis is more in line with your (1), that plasticizer is the major absorbing component of the plastic, so to make RADOME plastic you just leave out the plasticizer and live with crumbly plastic. But I'm no polymer chemist, so that's really just speculation.
I bet there are a bunch of polymer chemists who know exactly what's going on.