Are you seriously questioning whether or not the nitrogen cycle occurs? I can only go back to the 80s for my fish keeping experience, but that's decades so let's compare. There was cycling back then! Fish in cycling is still cycling.
Bottom line is, you need to do some research. Sorry, but it's true. Run some tests and you can watch a tank cycle yourself. Do you have a test kit? If your fish were dying from ammonia or nitrite poisoning then all the UV did was kill the bacteria bloom. You did nothing to solve the underlying problem.
I really have no clue if a UV sterilizer is merited on a freshwater tank. I've been fine without. The issue I'm having is that you think cycling of any sort is myth.
Sorry just my expertise and general knowledge. LOL
Cycling is not a myth. Seems like it is a "catch all" for every problem there is. I had my tank cycled in two weeks. Then I got "new" information from forums, etc. The magic water tests. They are ok. But again, not a "catch all". Did some "recommended" stuff for test results.....hell broke loose.
Now I had to recover from all those suggestions, expertise, and general knowledge, LOL.
Or my fish did.
Fortunately I keep on going. Discovered aquarium UV sterilzers, and that worked...so far. I don't know the long term effects of this, but will find out. I keep mine on all the time now. I was thinking of putting it on some sort of timer to just "periodically maintain" nasty stuff levels in my small aquarium.
The UV is fixing something. But I don't really know the exact cause of the problems it fixed. But so far the results are impressive. No fish losses. Fish seem happier, if that is possible.
I suppose a UV can be fix fix for some bad owner habits, but I have to fine tune myself.
My aquarium knowledge is out of date a bit. But I am learning again. Got my first BS in two years, so I can learn pretty good. I love to keep and observer creatures. I am limited, being disabled. But I have a dog, five birds two guinea pigs, and now fish.
I have found that on this topic that written documents online by experts in the field, and the bibliographies of their articles are far more helpful then forums now.