In my opinion ich and various diseases will always be present in 99% percent of our tanks and it’s about management over eradication. 76 days is a very long time to fallow and it’s not foolproof. Humans make errors…such as using shared equipment or having the two tanks too close. So I personally like to do an observation period but I’m going to be honest I think we all have skipped steps in the quarantine/treatment process or not done it all especially in the beginning. But in my experience and in my opinion unless a significant stressor or event stresses the fish and flares up the disease they can live happy and healthy lives for years with a good diet, maintenance, UV sterilizer, and low stress. So my thoughts are most fish/tanks have it and by treating them before they go in the tank in theory you can irradiate the disease before it touches your display but you have to do it for every fish, coral, and invert from the very beginning and even that isn’t foolproof and things you can’t treat or medicate carry diseases too.