First of all, groundless generalization is senseless. We know no statistics of how many survive into adulthood and exactly what people do with them. I am yet to see a single adult Chinese Hifin for sale anywhere, including on CL, here in the MFK Marketplace, Aquabid, Ebay, LFS, on-line vendors, etc.!!!
No one in their right state of mind would kill a 2'-3'-4' Hifin just because they can't house them anymore or because they lost their juvi colors. First, the conscience. Second, the attachment. Third, mister $$$. If anyone kills theirs, it's most likely because they need to up their meds or they are the sadist kind.
Second, many fish go through this process, e.g., Distichodus species, Leiarius pictus catfish, etc. If you don't like what your fish looks like anymore, find a new and right home for it. You are lucky it is not a cat or a dog or a parrot but the responsibility is still the same - whatever living being you took in, you take responsibility for its well being. Period. Next time do research and don't get the fish that you won't like for its lifetime. It's not a pet's fault you commit irresponsible choices and actions.
Genetic engineering brings about an unfathomable amount of suffering, some already evident, other is yet to be discovered and shown. In an analogy, let's say there are way too many people of the planet and the nature is suffering, the ecosystems are on the brink of collapse. We need to start culling the people or genetically modify them so they eat less, have lesser needs, die sooner or are born sterile.
Anyone who shoots dead a hitler like that would do the humankind a favor.
I'd say lots more but I'll step on my throat now.