I don't really care what salt one person uses or doesn't use, but this argument is a bad argument. Huge aquariums use a certain salt for two reasons. One, it's actually available in pallet loads and huge quantities on a consistent schedule - And two, it's cheap.
GM has been around for a million years too but that doesn't mean they make the best cars.
I'd add a third reason why big aquariums use certain salt - it performs well at keeping tens or hundreds of thousands (or millions?) of dollars of livestock alive and healthy.
I get your point with the comparison to GM but I don't think that's really valid. Cars are inherently emotional purchases driven by qualitative feelings. I like the way this car feels, I like the seats, I like the touchscreen, I don't like the color, and so on. It's a complex process and there are naturally lots of different brands at different price points, because there is legitimately a wide market of consumers.
Salt is very different, which was my point. It's a handful of basic chemicals in a well known proportion. There's no magic, there's no feelings, there's no qualitative aspect. You mix the salt with pure water and you get an end product that falls within a reasonable range of parameters.
I could see an argument about buying one salt vs another because the parameters we care about for coral (say, Ca Mg and alkalinity) mix to the desired target you want, but even that is weak, IMHO. Anyone who cares about those parameters is likely dosing their display tank, which means it's trivial to adjust the newly mixed water to your target, and likely at a much cheaper cost and with a known and very specific end state. I can mix IO and dose it to whatever parameters I want, and it's still in the realm of a third of the cost of some of the boutique salt brands that come and go over the years.
Let me restate my earlier post. If Instant Ocean (or any other cheap, mass market brand) has the correct elements in roughly the right ratios, is stable and consistent, is readily available, and has decades of success in the market, why would anyone pay two to three times more for something else? If a cheap product doesn't meet your criteria, then fine - don't buy it. But I don't think that's the case here, if your criteria are "a salt mix that can reasonably match typical target values, especially when supplemented with typical dosing regimens."
Sometimes, people in this hobby have a way of over-complicating the most basic things.