a single malfunctioning device will take down the entire network?
Bah. True of most networking technologies. A simple NC relay/switch permits a host to opt-out of the ring.
RS485 has pretty much done the same thing
Yes, that's more likely, and rs485 is a potential solution to the original poster's problem. Unfortunately, rs485 converters and/or ports and/or drivers seem to be particularly expensive, while the ring network can work with traditional (cheap, common) rs232 drivers or even just ttl levels. (it will require a common ground on all the nodes, so it may have problems if the nodes are physically far apart...)