Good post Lorenzoid.
My personal feelings are that for many it's not just personal amusement alone. It's also the challenge of being very successful. This is seen more with reef tanks rather than fish only tanks. For most who have done their due diligence and research and/or those with experience, the latter is actually pretty easy. There's not much of a challenge to keeping a very healthy fish only tank. A reef tank including corals, especially scleractinian corals, is far more challenging. Even still though we have become very good at it. So good in fact that very little coral needs to be harvested from the ocean anymore as we are producing such large bio masses of coral they must be trimmed. Much like you trim your garden hedges. Fortunately with coral, those trimmings (frags) can simply be sold, given or traded to someone else and will continue to grow.
One great discovery we made in the reef keeping hobby is that we can actually train, for lack of a better term, coral to become hardier in a short period of time through short exposure to periods of less than favorable conditions in an aquarium. So consequently there are some organizations that are using this technique to reproduce what they label as "super coral" to withstand the pressures from human activity. Again, how cool is that?
It's not as black and white as many portray it. We have huge forums of people sharing information. Learning and educating others. Just a few decades ago it was impossible to keep most hard corals alive for any amount of time in captivity, now we can keep them indefinitely and we're creating super corals that will help save the reefs.
The moral argument in terms of aquarium fish is one that's hard to say. If I kill a fish for consumption, it's dead. That's it. However, fish in an aquarium will most certainly display signs when they are not happy. Of course we can't ask a fish if they are happy or not living in captivity, but I believe most are oblivious or adapt very quickly. A fish that is not happy will most certainly be an unhealthy fish. Also, it should be noted a fish in captivity in today's aquarium keeping could very well be living in better conditions than in the wild. Great water conditions, no fear of predation, no exposure to new parasites and zero effort to acquire food. Note, I'm not saying that's the case in every system, but it is for the majority.
One final point, I recall a study conducted by the NOAA on Hawaii's fishery that actually made (whether intentional or not) a positive argument for collection of some ornamental fish. If I can find it I will post it here for reference. Nevertheless, due to shark fishing they noted a large balloon in certain species due to less predation. The problem with this is these species that are ballooning in numbers were pushing out other species that required the same resources. Ornamental collection has helped keep the balance withing the ecosystem since the apex predators have been reduced so much. So again, it's not so black and white as many believe.