I have also interacted with a lot of teachers online, and very few of them have struck me as being good.
I think my current teacher is good and does observe and teach well. However, he is quite literally one of the best in the city, and most students would not be learning from such a teacher. I learned more about technique and practice methods from Graham Fitch, Josh Wright, bernhard, C. Chang, etc. than from the other teachers, by and large. I promised myself that I would get a teacher once I could attempt Chopin etudes, and I'm happy with my decision. Which is why I keep saying that, yes, if you want to reach the very top level, get a really good teacher (typically who is at a level to teach conservatory students). However, many people, can not afford or access such teachers, and I was one of them for several years. That is why I keep saying that the limitations aren't as bad as people make them out to be, if you can learn in the way I mention while being cognizant of what you're doing at the piano. Perhaps not everyone has the level of introspection needed to honestly critique their own performances to improve, and read and understand technique and music theory -- in that case, I am talking to the significant minority of people who can.
As to the level of technique I was able to achieve, my teachers have told me that it's similar to what a child would achieve if they were not exposed to proper technical instruction with an good teacher, and I thought that was a good way to put it. It's not as good as the conservatory students I've seen who have been practicing for hours each day since they were young, that's for sure, and I think I know where the deficiencies lie. But I can play better than most people I encounter who aren't on a professional track to becoming a pianist, regardless of the age they started and whether or not they had a teacher.
So, it's possible to learn technique on your own, and many teachers will not be able to teach you better technique than what you can learn from videos by Graham Fitch, etc. You will not acquire a virtuoso technique, but you wouldn't acquire that with a subpar teacher either, mostly. That's been my experience.