We don't really even need altcoins to handle this. Just plug in another private key encryption method into the client, one that's resistant to quantum computing, whenever it looks like QCs start to become a viable threat, and have everyone migrate to the newer addresses (or not, their choice.)
That would be a good, however not a 100% safe way, to find out, if coins are lost or just sitting in a "cold" wallet. Then who whants to loose his wealth, because they sitting in a old, unsafe wallet without the proper, future security features ? So the addition of new features will indeed show, if the coins are lost or just not used. And additions will come for sure !!