There is no problem debating/discussing/arguing important aspects of home aquaria. Many inexperienced members new to the hobby come here looking for advice. The internet and forums like TFF can be the difference between life and death for fishes keep in aquaria. But where you and I do disagree is that you allow what I will not, because I understand the impacts on fish of everything that is outside the species' preference, and I recognize that even one small factor can be significant to the fish and to the member's stay in the hobby.
Manufacturers frequently say "x" product is "safe," or using up to five times "y" conditioner will not harm fish. Such statements are not true, and their definitions of "safe" are too narrow. The fact that fish do not turn belly-up when we use "x" or overdose "y," or the fact that outwardly the fish appears the same does not mean it has not been seriously compromised, and that can lead to further problems that otherwise would not occur.
All of this adds stress to the fish. Stress is the direct cause of 95% of all aquarium fish disease. That alone should tell us that every additional thing we do that is making the fish's normal internal functions have to work just a tad harder is without question going to impact the fish.
Fish have an absolutely unique relationship to their aquatic environment, much more crucial that that of any terrestrial animal to life in air. Once we understand that biology, it is easier to see why those few grains of salt do matter to the fish. It is impacting the fish's physiology and that weakens the fish. Along the way, other factors may do the same, until it is too late and the fish "mysteriously dies." It is not mysterious at all, if one realizes the steps.
As for re-mineralizing water, that too is not true for all species. Soft water species generally do not require any minerals in the water. I already mentioned that my water is zero GH/KH and I have had fish for over 30 years. Ian Fuller says he uses RO water with no buffering for his wild cories, and they are thriving. As you move toward moderately hard water species, there is a need for mineral salts. But those are salts of minerals like calcium and magnesium, not sodium chloride which is detrimental. If the aquarist needs to re-mineralize the water for the species being kept, there are safe methods that only provide what it needed. And that will always be safer for the fish. And if using Instant Ocean to achieve the mineralization necessary for these species, the level of sodium chloride would be very high as it is part of the mixture.