There are simple facts in the hobby, I feel like on of the "inconvenient truths" is that if you don't QT everything, eventually, it will bite you. Say there's a 5% chance of this one thing and a 50% chance of another more common thing, you roll the dice each time you get a new critter. Some people are lucky and go years without issues. Others kill all of their fish regularly.
The other issue is, and pertinent to this question, if you don't QT ALL critters going into the tank, you will still likely get things like ich given its free-swimming and settling stages of life. This leads many people to throw up their hands and say "Jesus take the wheel I am not quarantining anything!"
There is also the issue of coral pests that regularly get through coral QT which is maddening.
The long and short seems to be, if you have room, and utmost dedication to health and wellbeing of your critters, then a rigorous QT of all animals is the only surefire way to keep a DT clean. But anything less than 100% leaves you exposed to some small degree.
I am not sure where I'll fall on this in this go-around. One way would be to introduce clean up crew before fish and run it fallow for 7 weeks just feeding algae and CUC while you QT your fish then add them when out of QT. Man that seems like a lot of work and waiting, but is it what is needed? And what happens when you need to add more snails in a year? More snail QT?
In the past I didn't really QT, just kept fish all fat and was by and large successful. I don't know if that makes me irresponsible or normal. I know we get cats/dogs and have them de-wormed/treated. Why wouldn't we do that for fish? Interesting discussion