Yes, if you left a plastic framed gun sitting outside in direct sunlight for 10+ years, it is possible that the UV radiation might degrade it to the point that it becomes brittle.
However, I work in an industry that uses plastic parts that are continuously exposed to weathering and direct sunlight and we warrant our product for 10 years. Carbon black is great for UV resistance.
In a weapon left exposed to the elements and sunlight for 10+, weathering would also have rusted the gun to near uselessness as well, even if it had an all aluminum or worse, steel frame.
M16's have been using plastic grips since the 1960s in military service. The HK M23 has been in service since the early 1990's, I haven't heard of any of them disintergrating.
I don't think that the plastic frame is anything that would keep a weapon from becoming an issued weapon.
The SCAR has a plastic lower, the AUG has a plastic frame and even plastic trigger components and it is has been in service since the 1980's.
I am no longer participating in gun forums.