Re: old fish in nitrate-heavy tanks, I think it's just survivor's bias.
The oldest person ever documented is Jeanne Louise Calment, who died at the age of 122. She smoked well into her 110s. Should we conclude that smoking has no effect on your health? No, because plenty of other smokers died earlier than they would've otherwise. So it goes with nitrates: there might be some fish that can endure it and live to old age, but it's only because they won the genetic lottery, and there's no knowing how many died early because of it.
Also, we don't really know the maximum lifespan of most fish. A decades-old animal sounds ancient to us, but that's not because it exceeded its natural longevity, it's because few fishkeepers have the dedication, resources, and sheer luck to keep fish for that long. You might have to give away your fish because you're moving, a disease, power outage or cycle crash might decimate the tank, a fire or natural disaster might hit your home. If we all had access to ideal conditions and infinite time, we might learn that fish regularly get to be that old, and perhaps even older if kept in nitrate-free conditions.
(For this reason, when you look at internet resources for estimated lifespan, you'll uniformly find "5-7 years" for small fish and "10-15 years" for larger, more robust species. This is bogus, we know that larger plecos, synos and loaches can live to be thirty. But few keepers have these fish and the information isn't widespread).
Lastly, I've heard an urban legend that if your nitrates are above 20 ppm on Halloween night, a mysterious spirit will manifest in your fish room, show you nitrate tests he took from the natural environment of your fish, and drown you by pushing your head into your tanks. Halloween is soon. I am not taking any risks.