Experts agree that you should wear your shoulder strap along with your lap belt when riding in a car, for safety. Does that mean that if you wear your lap belt, but put your shoulder strap behind you, that you basically aren’t wearing a seatbelt? Even with just the lap belt in place, your survivability in case of a car accident is much better than with absolutely nothing. Sure, it does happen that people are eviscerated because the shoulder strap that is behind you pulls upward as it is intended, and can end up slicing your stomach open... however those cases are not common.
It would be hyperbole to say that refraining from wearing your shoulder belt is akin to not wearing a seat belt at all... because you still are more protected than if you wore nothing.
The same is true regarding QTing of fishes and not inverts. It is hyperbole to say that if you do not QT inverts, it is akin to not QTing at all, because even just QTing fish, you are more protected than if you were to do nothing. The percentages of those protections are, as far as I am aware, unknown... but it is widely accepted that fish are far more likely to carry ich and velvet, for example, than an invert.
Are tanks wiped out by an infested piece of live rock? Certainly, as you can attest to. But, the generally held view is that is far more likely to happen with a fish that hasn’t been quarantined than an invert or something else that is wet.
I do not disagree with you that risk is incurred in not QTing everything wet... I disagree with you that QTing fish only completely erases any protection gained, and that it gives me the same probability of being wiped out by velvet that I would get if I went to my local PetCo, purchased a Hippo Tang, and dropped it in my DT the same day.