A LOT of people living dangerously. I guess everyone needs to find out the hard way before they start QT'ing fish.
20 years ago, with my first saltwater tank, I never QT'd any fish and they were all fine. Nowadays, fish come with a LOT more disease. I blame mostly the wholesalers for this. I believe they used to treat fish before selling them to a LFS. Now they don't.
After taking a break from the hobby for 2 decades, I started this time without QT'ing. It didn't take long before I was treating my display with copper for Ich.
Now everything is QT'd, but I keep it simple by using a small tank, 1-2 fish at a time (tops), and treat with copper first for a month, Prazi for a week, and then the triple threat of antibiotics. So fish get to my display 100% disease free. They also get a freshwater dip before going into QT. Haven't had 1 problem doing it this way. It's just the world we live in now.
And to top it off, once the fish goes into the display, I use an introduction box to keep the new fish isolated in the display for 3 days. It gives the other fish a chance to get up close & personal with the new fish without contact. I've even done this method with fish that shouldn't get along and after the intro box, there was little to no conflict.
IME, this method is rock solid from all angles. I rather QT properly first in a small tank than treat the display again.