I tend to lose few fish, it's often some years between losses-- though if I say 'it's been x years since I've lost a fish' sometimes that's when it happens. Don't know if that's Murphy's law or some other law, lol.
Usually, I know the cause. There's old age, which you can often see coming with an elderly fish with certain signs and I tend to euthanize when they get to a certain point rather than let them continue to break down toward the inevitable. I've lost a few kapampa giberrosa females over 13 years to being egg bound (something they're susceptible to). There was a time years ago I'd lose the occasional Malawi cichlid to aggression or bloat-- until I figured out how to deal with and prevent both, which fixed both issues. Once lost a couple of guianacara when pH went too high in their tank, forget what I did wrong but I figured it out and fixed it-- interestingly, red head geos in the same tank were okay.
Occasionally I've had a mystery death, rare in the last 20 years, but it happens. Usually I knew when something was off, saw it in their behavior and tried treating. Less often a sudden death with no warning. It's possible some cases were flagellates, but there are other possibilities and, as you say, without testing who knows?
I believe in bio-security, especially in what you feed, in observing and knowing the behavior of your fish so you recognize when something's off and can address it early, in sound nutrition and not overfeeding, in not using antibiotics unless you need them, and in understanding your tank as an eco-system-- water changes are fine, but they're not the end-all-be-all of a healthy tank, tank ecology matters.
