Well, of course you are right. The problem is that for many people (including me) it is hard to use it in a way that actually brings value, because it is just too easy to spend hours on there without getting anything in return.
I also think that it is highly subjective what can be considered "good" or "bad" content. When it comes to educational content, I also would consider it a waste of time. Sure, if I have a real life problem and the solution happens to be described in a youtube video, there is nothing wrong with watching it. But often times I was just like "Oh, this could be useful at some point in the future" and at the end of the video I could hardly remember what it was about. I also don't think that "mindless" content is inherently bad. If it helps someone to relax, go for it. I always felt worse afterwards.
Saying youtube would be a big waste of time in general is indeed reductive, but I think for many people it actually is, because it is just not designed to be used to bring you value. The only objective is that you spend as many hours on their platform as possible.
Even with an ad blocker it's insufferable though. Every time a page ask me to sign up for their stupid newsletter I want to punch a hole in my screen.