Ich is not the biggest problem parasite, it is the least. ...
^^^ This. Ich is more of a nuisance than a threat. No one should loose a fish to it, it's just way too easy to prevent or treat. All the losses I see here come either from inexperience or from people who have tanks that are too large for the available support (QT/HT) capabilities to hold and treat all those fish out of the DT.
The things that scare me are velvet, uronema, vibro and other bacterial infections. Intestinal parasites can also be a pain to remove.
In the past I did not treat ich at all - I simply never had a tank wide outbreak, only single new fish or fish that got bullied getting a few spots. I just identified and fixed the problem and the spots went away (the fish's immune system took care of it).
Now, back then I cherry-picked my fish and made sure no new fish had any signs of an active infection. Also, new fish went usually into a new tank I had set up for them (back then I was collecting clownfish pairs and other pairs of fish for breeding).
Most my tanks were small (below 150 liters) and had only 4 to 6 small fish in them - the clownfish were usually the largest.
Also I did not have any tangs or the like (which are IMHO among the most ich prone fish you can have in a home tank).
And most importantly - I bought almost all my fish directly from a high quality importer, ideally only a few days after they arrived.
If there is one thing I learned, then it is that in 99.9% fish don't come with an ich infection from the wild - they acquire it in the wholesale or local fish store systems. So the sooner you get them out there the better.
These days I'm a bit more careful since all stores around here have some ich, or brook problems going on. All new fish get a formalin and freshwater dip and then get quarantined for 2 to 3 months and treated as needed.
I still only treat ich when I see an infection and even then, only when it escalates.
However, all fish will go through TTM at some point before entering my new tank - but that I consider prevention and not treatment.