a 'fluke' - no pun intended

....
I personally think a 75 day QT - observation - where there is no evidence of disease, the fish are eating etc etc - is suboptimal but may be 'ok'. With a couple comments:
1. Its still not a guarantee - as CI, velvet and other things can be present but not visible in small amounts - and can pop up later. some can also be passed back and forth at the same time - and not be visible.
2. Its stressful for the fish to be in QT (depending on what kind of tank you are using) for 75 days. Most people would do an observation-type quarantine for 2-4 weeks.
3. If you do a copper and perhaps a de-worming medication at the proper dose, with the proper aeration, the proper levels, etc - it seems like it should be far less 'risky' than a 75 day QT with nothing.
4. If your QT tank contains live rock etc - after the 75 days it will have to be left fallow for 76 days - before adding new fish.
If your afraid to QT with medications on your own I would highly suggest that you go to one of the sites that pre-quarantines their fish - and THEN do your observation for whatever time you like. Then - you're not using the medication - and still doing the observation
Hope this helps!