One reason there are so many surviving Prestige rears is that there were so many of them in the first place. Bikes that barely ever got ridden probably account for the survivors. They did work well when new and didn't wear out all that quickly, but too many of them failed catastrophically, which is no fun at all. A cheesy metal derailleur has its own problems, but unless you drop the bike on it or send it into the spokes it will probably not break in use.