As an owner of an '85 Wing (33 years old) and a 1997 Wing / '98 Lehman Trike (21 years old) I gotta say I have yet to see any evidence of the plastic (most of it ABS) being / becoming "brittle". ABS is used in a lot of buried plumbing intended to last. I have broke a couple things, but that was my fault. I don't plan on wrecking either and I bought each to ride with little thought to resale (bought '85 in 1995, bought the '97 in 2004).
The one time I hit a rock in a shadow with front wheel kicking it upwards , and found need of a couple parts (lower cowl, left exhaust shield), they were located that eve on E-bay cheap from a salvage yard in Fla., were even correct color.
Besides, the forces placed on a trike frame are somewhat different than a two wheeler sees, 1200s & 1500s have steel tube frames, somewhat more resistant to fatigue. Not saying an 1800's frame can't take it, but I don't recall any frame recalls for the 1200s or 1500s either. Time has shown, both the 1500 and 1800 make fine trike bases, both have their strong points.