I find most fish deaths when you've had a fish for a long time, kind of come on this way, sudden and without warning. I find in general with animals, like even with cats and dogs, it's just a sort of built in instinct to not let on that you're not feeling well and when it starts to show externally that you're not feeling well, you're REALLY not feeling well.
Five years is a remarkably long run in captivity with a copperbanded butterfly. It's doubtful that their livespans are this sort in nature, but you have to also take into account that while the natural lifespan maybe longer, lots of individuals won't make it out in the ocean either due to predation and so on. So it's hard to say what an "average" lifespan might be, but rest assured that five years is a very small minority in captivity. Most aren't able to get them beyond a couple of months and there is a lot of talk out there about this particular fish being perhaps better off left in the ocean since so many fare poorly.
I myself thought I was doing reasonably well at close to 3 years and then poof, I noticed he wasn't quite right one evening and 3 hours later he was gone.
Anyhow, I feel your pain, and I am very sorry for your loss.