No doubt they are weak boxes but im sure one in decent condition will last a long time. With 30y old gear it really depends on condition and how much usage it has has. Take my old h145 diffs for example. Original one was always a little noisy and let go after 12months of hard abuse. Chirp second, snap second around corners, line locks etc and it was stuffed in the end but still drove fine. Put in a replacement. Lasted two weeks. Chirped second like five times in the two weeks and it smashed to pieces on the frinal chirp after about day 13.
My orinal 56A box still lives though behind the worked a14 though after 18months. Synchros are now a little noisy but never sounded crash hot in the beggining.