This is a rather unfair assessment due to the fact that the aquarium industry is not able to replicate the vast richness in food concentrations available in their natural environments;
It's a very honest assessment of two decades worth of observation as well as advanced studies in the biological sciences To understand the 'bottom line' to what I am saying you would need to be familiar with molecular biology, chemistry and the role of proteins and their formation.
Considering all butterfly fish do eat coral, some not exclusive as others are, but all do nonetheless. This means there is an essential protein to that animal's health, longevity and reproductive capacities that without that protein, would end up failing over time. Often leading to premature death...even if the fish lives for 5 or 6 years...it can well still be quite premature.
The results of an absence in coral protein in their diet seems to be accumilative (such as like how increasing nitrates are in a tank). Problems may not occur until up to many years have past in accumilation to this deprivation of an essencial protein.
In my honest opinion, butterfly fish are really poor fish for captive care and best left in the ocean.
And...BTW...we CAN replicate "the vast richness in food concentrations available in their natural environments". Coral are amazing creatures when it comes to replication and most can do quite well in captivity. They are very propagatable.
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.