your combining two issues, i wasnt. when ever the question of on line courses comes up there is post after post about how people will injure themselves. there is an uproar to the point that you would think there was a good probability of death.
can someone without prior experience get proficient at martial arts from watching a video,,no. i do think there is an exaggerated focus on people getting injured by doing so. i would bet the biggest injury would come from thinking you know how to fight and finding out otherwise should you find yourself in that position. but other than that people do and have been doing physical activity without good instruction since the birth of man kind. its not a big deal.
I tend to agree, but like everything, it depends.
When I was young, probably under ten years old, my brother (one year older than me) and I had a book on judo. We went into the basement where we had a play space, rolled out a thin carpet over the concrete floor (that ought to be enough padding) and practiced our judo throws. I landed on my head.
Now that could be just the ignorance of the very young, or it could be a stupid move that a more mature person might make without proper guidance. At any rate, we didnāt know how to throw properly, we didnāt know how to fall properly, and we certainly didnāt know what appropriate floor padding looked like. And I got hurt for it.
So it depends on what one is doing.
Mimicking kata? Probably not imminantly dangerous. Practicing throws? Yeah, definitely dangerous.
Hitting a heavy bag? Potentially hazardous.
Practicing with weapons? Possibly quite dangerous.
Two idiots having a sparring match? Someone might get hurt.
The other thing is that improper technique can be dangerous, but I would say that is an injury that takes time to develop. The way we practice the fundamental punching technique in my system will tear up your shoulders if you do it wrong. Without proper instruction, I can guarantee you will do it wrong.
The way people tend to throw out their foot in a kick, will get them a broken ankle if they ever land it on a heavy bag. And their hips can suffer over time, from side kicks and roundhouse and hook kicks. Best to get good instruction to minimize that possibility.
So yeah, people have muddled through physical activities without good instruction since the dawn of time. But some activities can hurt you, if you do them wrong.
But I tend to agree that the biggest issue is simply the self-delusion of developing crap martial arts and bad habits, while believing that one is developing good skills.