A hay bale of thoughts (random and not all of the same quality) .....
QT is simply the process of isolating a new fish from the established population. This, in and of itself, is not harmful to the fish. Whether one chooses to observe in QT as opposed to prophylactically medicate is a different question. I do not personally agree with the recent trend (last 5 years?) of blasting new fish with a cocktail of meds as I do think it is injurious to the fish; and in ways that may not be immediately obvious. Fish may not die, but it also may not live as long; so I don't do it.
I keep a smaller 29 gallon tank with live rock, sand, skimmer, a good light .... well, you get the idea. New fish go into this tank for initial observation. If, and only if, disease symptoms present then I remove it to a hospital tank. Even though there are anecdotal reports/opinions on the higher amount of disease in the ornamental fish supply chain, very few of the new fish I buy actually end up needing any kind of treatment. I haven't had to treat for ich in a couple of years, and have not had velvet for over a decade. Why is this? No special magic chez moi, so perhaps we don't give fish enough 'credit' for their immune systems.
My approach requires patience because a new fish will spend at least a couple of months in the 29. If observable disease were to occur, then it would have to remain fallow for the requisite period of time. Both necessitate patience

. I will not put a new fish directly into my display, mostly for disease but also because running the tang gang gauntlet is not conducive to getting a new fish to eat.
I also run an oversized UV on my system. Not because I think it will eliminate parasites, but because it will (may) help to reduce parasite pressure (rather like ocean dilution) should anything slip through.