Enough that someone made a "top 35" list
The 35 Most Brutal One-Punch Knockouts
I believe the shortest UFC match was ~5 seconds ending in knockout.
If your point is "relying on getting a one-hit knockout is foolish", I agree. If your point is that they are extremely rare, I think that's overstating.
I have a feeling that if we created a new sport where we added knives for everyone we'd see something similar.
Heck, if we just changed the existing floor to stone it would have an adverse effect on many popular techniques. I remember a reality show from a time back that had a bunch of MMA guys in the same house. A fight broke out. One went for the... is it a "standing arm bar"? The one where you grab the wrist with your ands and pull it across your chest, but with a standing opponent meaning you are putting your head on the floor)... his opponent simply pulled him up and slammed his head on the concrete before he could get the bar.
The technique was good, but the conditions were different and it was the wrong choice.
I recall a street fight that made it to camera with to South American MMA champs fighting some locals. One of the locals had a stick. He swung the stick at the MMA guy. The correct approach would have been to move in, but he didn't. I have to presume because his training is against unarmed people and backing up from a punch is appropriate. With this, it just left him in range to get swung at again, and he was, and he was struck in the head and lost.
UFC and other limited rules competitions absolutely exposed flaws in training methodology as well as fantasy approaches to combat; but the fact that people who trained with a specific set of rules succeeded better under those rules than those who trained with a different set is, well, a no brainer.
I suspect you and I agree more than we disagree here; but I do think your position is overly absolute.