However, stating that adding any salt to an aquarium is "unfair" to the fish is absurd.
It's not absurd. The addition of salt dehydrates the fish and causes kidney damage, leading to permanent damage or death. However many fish reach salt naturally so they are not the type of fish I am talking about. Sweetwater, or fish that entirely in fresh water is what I mean. Most cichlids should not have salt, even if they are hardy enough to live through the effects. Fish like mollys, guppies etc. can live in BW. ( fun fact, guppies can actually live in water up to 200X the salinity of salt water. Obviously not recommended though. )
Your fish should easily reach wild size and exceed wild lifespan - then you are doing it right.
I used to believe this as well but there are factors you are not considering.
One, room. No hobbyist can provide there fish with the amount of space the fish will have in thew wild. Therefore they are not swimming as much as they would be in the wild and therefore do not put on as much muscle which can stunt growth. ( not in the same way bad water quality does mind you) Two, genetics. Many species of fish, alligator gars included, have been bred specifically to be smaller than their wild doppelgangers. These two facts along with other factors ensure that fish rarely if ever meet there natural size.
Is this not an alligator gar? Looks like a 10' fish to me:
That fish is no where near 10'.
Also ( sorry for the super long post) the largest gars in the world HAVE reached 10 feet. The usually don't surpass 8 and even then that is a huge gar. In captivity they reach 5-6 feet MAX. Usually only 4 feet. I have seen 8 foot gars at a public aquarium but they were wild caught and had been raised up from 4 feet. Those are the largest known gars in captivity right now.