DiscusPaul, first thank you for providing your guidance. I've grown several beautiful groups of Discus, mainly following the advice you have set forth on this forum. I got started about 5 years ago, and any time I run into something I don't know, I look to see if you've addressed it on the forum before I look anywhere else.
I would like to suggest a few concepts as they relate to the "deeper" reasons that discus require frequent water changes, as it is apparent that many out there are, at the least, curious. Before I begin, I would like to make it known that I am not simply guessing, as I have many years under my belt as a student of biology, both human and wildlife. Furthermore, I do not possess data (nor did a quick google search return any) revealing anything regarding discus cellular physiology explicitly and their ability to adapt. But, based on pages and pages of text regarding how organisms on this planet adapt, and years of delving into aquatic science, I believe the explanation to be thus:
Discus, as has been stated, come from a source of water that provides constant turnover. As such, there is a near infinite supply of needed gas exchange, nutrients, and waste removal (among other things) by nature that try as we might, we cannot imitate perfectly as aquarists. This by itself seems to make the necessitating reason for water changes apparent- because aquariums are closed systems (in most cases). We have to intervene to remove the waste. Otherwise, the organic/chemical dilution of the water becomes such that a discus cannot thrive. Again, I do not posses the cellular knowledge to tell you why the discus specifically does not thrive, but I feel confident that the process is likely akin to humans trying to breath at high altitude. Sure, there is oxygen at high altitude, but in concentrations our bodies are not accustomed to, along with higher concentrations of other gases. Sure, we can adapt to a certain degree with an increased hemoglobin/hematocrit, among other shifts, but only within a certain range. Insert any other metaphor here for trying to breath anything besides O2 or drinking things containing your own waste. Therefore, not enough O2=not enough ATP production for cellular energy=ultimate cellular demise (oversimplified but you get the point).
Now, to address the discus adapting from native water quality to the tap water that many of us keep them in. Ultimately, this ability to adapt can be blamed on natural selection. Whoever the first person was to try and keep a discus in a tank, I would assume had trouble doing so. I would also assume they had more success the more closely they tried to imitate the natural environment of the discus. Once that was a success, eventually, someone figured out how to get them to breed. When the fry hatched, I'm sure some perished, but the ones that survived obviously had a more tolerant genotype in regards to water quality, likely partially inherited from their parents. This went on for generations, along with chance mutations, with the survivors passing on their hardier genotype and so on, until we ended up with discus that could acclimate.
No, this does not explain how people successfully keep wild caught discus with tap water, but it is fair to say that discus still in the wild must possess some degree of physiologic adaptability, otherwise they would have become extinct years ago. Darwin's theory (while controversial to many) postulates that the longer a species survives, the more it adjusts to its climate and becomes more tolerant of what nature dishes out (or it ceases to exist). So, the longer discus exist in the wild, with fluctuating climate and conditions as they are today, the more tolerant the ones that propogate will become (in most cases).
I would not hesitate to apply these concepts to other species of fish as well. Of course, one may ask, why can some fish tolerate never having their water changed while discus (most of the time) cannot, despite the genetic adaptations? Well, again, we have to go back to the past, and its pretty simple- however they came to be (whether you believe in divine creation or primordial soup), each species was born of its own unique circumstances, therefore producing varying margins of genetic and physiologic tolerance in regards to pH, temperature, GH, KH, etc. through the process of natural selection and mutation.
Of course, this is fairly general and needs further testing. I don't have a bunch of discus lying around to dissect and see what may have contributed to their demise, nor do I have the funds or resources necessary to conduct longitudinal studies regarding how they execute glycolysis or the like. But, I hope the above information at least helps with everyone's understanding, and that those who are naturally inquisitive now have a bit more information to rely on regarding the "laws" of discus keeping.
Please feel free to add or correct if necessary.