But there's a big difference between playing basketball and truly understanding quantum physics. A 5 year old can play basketball but will never understand quantum physics. Very few people (even trained scientists) really understand it. Heck, our brains evolved to figure out how to catch our next meal while avoiding being something else's next meal so there's no reason to believe we have the capability of truly understanding quantum physics. The best we can do is develop mathematical models that appear to work. The majority of people probably could not understand the models even if they had the training.
THen quantum physics is a bad example, being the most advanced possible version of a thing. That's like insisting that all spellcasting is comparable to playing Hockey at the level of Wayne Gretsky. No 5 year old will ever do that, no matter how far into the future the human race persists, but they can
play hockey.
Every person over the age of 10 with no cognitive disability can learn to understand fundamental physics, even how to apply them in practical scenarios.
If learning to cast any spells at all in an arcane framework requires the sort of mind that can grok quantum physics and begin to advance it's study, then the disconnect between mid to high level arcane spellcasters, hell even a 3rd level full caster, and every other mortal being that exists is so great that the game is unplayably absurd. Nothing in the world of dnd can then be understood meaningfully, and no discussion of it's particulars can ever have any validity of any kind.
If, instead, we assume that only the highest reaches of arcane understanding are equivelent to the highest reaches of scientific understanding, the game makes enough sense to do something with. One equals one, and added to itself becomes two, and thus we can discuss particulars and understand the fictional world to some meaningful degree.
We all have limits, I see no reason to believe that anyone at all could grasp the concepts of manipulating reality through magic. Is it 1 in 10, 1 in 100 or 1 in 1,000? Depends on your world and your assumptions.
We know absolutely that in a dnd world using the rules of 5e dnd, there is arcane magic below the threshold of what you keep insisting as the floor of understanding to be able to do arcane magic. Half the ways in which arcane magic can be gained become complete nonsense, otherwise.
It's a skill. Like all skills with real depth of application, there is a level past which the amateur simply cannot reach, but the fundamentals are a matter of drive and practice and access to information.
As far as magic items, the basic assumption has always been that once a magical item is created it's very difficult to destroy. Enchanted swords do not rust so that sword created 500 years ago that was lost at sea could show up and be perfectly fine after you scrape the barnacles off.
Okay? What's your point?