Either way my take on QT is this.
Are you able to tell how any of these fish died? I've got a freezer full of dead fish. None of them have visible signs of any disease. I keep them so that one day I can find out why.
Hint: No two groups were ever in the same tank. Some I've had for almost a year, some only a few days or a week. Some went through prophylactic treatment immediately, some not at all. Only one of these fish was in a tank where a hippo tang that I've had for 9 months with no new introductions all of the sudden out of no where got ich. That went against everything I've read about the lifecycle of ich.
My personal experience, not doing prophylactic treating fish in qt I get healthy fish. I don't care what any scientific study says. The moment I started doing more than observation QT I started losing a lot of fish. Maybe I'm doing it wrong. I followed and triple checked before hand so I don't believe I am doing it incorrectly but I'm am pointing the finger at no one but myself for these losses.
Would some of these fish be alive and healthy if I had only kept them in observation ? I don't know. What I do know is that I've lost more fish through prophylactic treatment than not. I've lost enough trying to prevent an issue that I feel sad that a fish loss doesn't devastate me anymore.
If anything good that has come out of prophylactic treating fish is that the stress of losing a fish takes less of a toll on me emotionally now. It has conditioned me to where I try and convince myself that these fish would have died anyways and taken out every other fish with them. The problem is that I don't believe any of that. Maybe this is how the strong advocates of QT feel. I don't know. It just sucks either way.
If I lose fish, I don't want to be the cause of their death anymore. QT is supposed to prevent deaths and that is just not what it is resulting in