I would agree with Nossie, I'd rather a fish be food for 5 seconds, than suffer in a 5 gallon tank for years.
The original poster asked about nitrates, us telling her those nitrates being too high will harm her fish, is honesty. Sky high nitrates aren't a huge deal, they are easily fixed with increased regular water changes. That's all it takes.
There's no need to try and convince them their fish will be okay, when it won't be, just like there is no need to try and justify a fish in a 5 gallon tank, when it's simply not fair to the fish.
You also cannot compare your life to a fishes life. That is just a crazy thing to try and compare.
Firstly that fish is in that 5 gallon tank, with bad water quality and it will become stunted, shortening it's life span. It would be like confining someone to a box room with bad air quality. It would cruel to do that to a human, so why should it be any different for a fish?
You may have a small house or a trailer, but you can go outside. A fish can't suddenly decide to go for a walk!
I appreciate your opinion, don't get me wrong, I too wonder what a goldfish would pick if he had a choice.
But as a pet owner, it is our responsibility, to provide that creature with a suitable and adequate environment. If you can't provide the environment, then don't buy it.
The only way a 5 gallon tank would be acceptable, is if it were temporary accommodation for a few months and the fish would be getting a bigger tank or pond