I have been in the hobby now for about 40 years also. Over that period of time I have had my Current Tank running non stop for the last 19 years. At it's peak I had about 12 fish, numerous shrimp and lots of soft and hard corals. Most had been thriving and growing for almost 10 years. My pair of Mandarin and my pair of Clown fish laid eggs so often it was annoying. In the end the tanks inhabitants were bought down by a Temporary Maid that came to clean the house for a day. She saw some ants near the Tanks base and decided that a half a can of pesticide from the closet would do the Job nicely. I came home about 8 hours later and only noticed an ever so slight smell of pesticide. By the next day one fish had died, by the following three more died along with the corals starting to look really bad. The whole time I had no clue what was going on. I did water changes and used Carbon packs but by then it was too late. At the end of the week all but one fish had died and almost all the corals were wiped out. On the following Monday she came again and admitted that she had sprayed the area to get rid of some ants. I told her to leave lest I do something to her that would put me in jail.
Anyway since that time I have had a lot of time to reflect on the hobby. My system was built in a very meticulous way with an insane amounts of automation and Triple redundancy for most systems. I even had the system on a 1500 Watt UPS system along with the tanks own Honda 3000EU Generator for backup power and In the end it was an idiot with a spray can that did me in!
I now believe that it is nearly impossible to keep an aquarium going long term. Too many things that one cannot predict can go wrong. Call them acts of God or whatever, but they do happen and no matter how knowledgeable you are and how well your equipment is setup and maintained these thing will happen if your tank is running long enough. I would say that almost every year something would happen but on almost every occasion I caught the problem and fixed it before any damage could occur. I also made changes so it could never happen again. Unfortunately it is that one out of ten times when you don't catch the problem or the problem just cannot be solved that will do you in.
Rob