Again, not enough specifics have been given to make such a blanket statement.
There is proof that most people will not need R/O water, and that proof is the hundreds of discus keepers(unless they are trying to breed) that do not use RO water. Most any tapwater will be fine for discus, that is a fact. There are very few instances that someone will need
R/O water to keep discus unless you are breeding them. It's pretty much overkill unless you are trying to breed or in extreme circumstances. For example, if your harndess is much higher than where you are getting the discus from, the discus may have a tough time acclimating and thriving(still can be overcame with careful acclimation). I absolutely admit though, that it is more ideal to keep discus is softer waters, just like other fish but it certainly isn't a requirement.
Ph of 8.0 does not necessarily mean hard water or even relates to what the hardness is. When I lived in ND my ph was only 7.6-7.8 but it would kill most discus and even domestic angels in a very short time. Why? Because my gh and kh was constantly in the 20's and even my tds reading was ovr 600. I could raise discus fry in the water up to about 3-4 inches but then had to start cutting the water with RO.
Your right, high ph doesn't mean hard water. But you would be hard pressed to find a tank with a ph of 7 or 8 without a trace of hardness. They are related. GH and kh can be in the 20s and not kill discus so long as they are acclimated properly and the parameters are stable.
As far as "Most Discus Breeders" not using RO...... Most of the ones I know DO or they have tapwater that is less than the liquid rock found in many parts of the US. I can also say that I've had far more wild discus than domestic and I find the wilds to be a lot tougher overall with only a few exceptions.
I didn't mean breeders, excuse me, I should have said distributors. There are plenty of middleman sellers(like Kenny, April, etc.) at simplydiscus, discusforums, etc. who buy from breeders in Asia. They then sell them to other people here in the states. And most of them don't use R/O, now do most of their customers. If you are breeding that is another story because the fish have more special needs. In regards to the wilds, if they are truly wild caught then you are in the minority, as tank bred discus(and other fish) are generally more hardy because conditions are usually more alike with tank bred fish than fish in the wild.
But in regards to keeping discus with no desire breeding them, there really is no need for R/O water in most cases. My water here in Cincinnati, Ohio is hard and my discus have thrived. Most have grown 3 inches in 3 months.
I won't say that discus cannot be kept in tapwater but I won't say they can either without knowing what the parameters are. Even then, the parameters will have a bearing on the maintenance schedule. I don't believe discus are as finicky as many say they are but if they could handle the same conditions as rift lake cichlids and/or livebearers the stores would be full of them.
I'm not saying they aren't sensitive to water quality, as they definitely are. But stability and clean water is much more important that harness and ph. The simple fact that most discus keepers aren't using R/O proves that it isn't a necessity for just about all cases. And the main reason they are not in fish stores is because good quality discus, as you know, can run anywhere from $100-300.
Discus are very much like blue rams and cardinals, with discus being more hardy than both when adult. They can be kept successfully in hard water, but it is recommended that they are in soft water(because that is their original environment).
But I am wondering at what point do you think R/O is necessary? What parameters would mandate R/O?