"So still no compelling evidence to upgrade, especially based on my driving
experience of 4 years each in an '85 and '84 Westy."

Probably cost.  Fabricating arms out of several pieces of steel and welding them were likely more expensive than casting. That's my guess.

I'd prefer the welded steel than brittle cast iron anyway.  Steel when over stressed will bend, the cast iron will break clean off, most grades have have very little ductility.   Cast iron, repeat after me:  strong in compression, weak in tension.

My $.02, your mileage may vary.

John R