great thread topic!
it would be interesting to have breeders breed with hardiness in mind versus looks. i know some of that happens anyway as most breeders will use their fastest-to-mature and most healthy to be breeding stock and usually that equates (somewhat) to hardiness.
i would think one could use fish that have survived some neglect/dirty water/pathogen exposure without showing too many adverse effects and keep breeding for that trait
however...i'm fairly certain this will not happen in a market economy as they would not sell as well as the ones bred for appearance/shape..it would take a dedicated hobbyist or a determinded effort by breeders with a long-term view to breed for hardiness, as cash-flow talks when people are trying to make a living with fish.
in time however, maybe would be possible to raise tougher (but also beautiful) discus, thus expanding the hobby to those that are not currently able to commit to the time it takes to raise discus.
interesting