> Some of the fastest and most famous guitarists had shockingly bad technique.
The universe didn't offer a manual on how to play guitar, so how are you determining that their technique was bad? Given what you say about them, maybe they actually had the perfect technique?
The universe didn’t offer a manual, but mankind has largely arrived at some orthodoxy for the most efficient and ergonomic ways to fret, bend, pluck, tap, strum, etc. In many cases, these are objectively better techniques to use once mastered, but they’re not the only way.
> In many cases, these are objectively better techniques to use once mastered
Given a certain set of quantifiable measures that is no doubt true, but then that only pushes the question to how are the measures determined to be objectively relevant? If the aforementioned fast/famous guitar players had started with a different technique there is a chance they wouldn't have become fast/famous. In that case, given the criteria of reaching notable speed/fame, it is possible their "bad" methods were actually best of all.
But also, even where everyone agrees there is a better way, that doesn't equate to an alternative being bad. So the original question still stands: How do we determine "shockingly bad" as opposed to "different"?
I see this "bad technique" angle expressed quite often when talking about self-learning. I tend to think it is overblown a bit. I started learning piano by myself during covid. Then I went to two teachers. Neither one had anything bad to say about my technique.
Teachers of beginners understand that keeping them motivated and practicing is the primary problem. Cleaning up your technique is simply not a goal. Doubly so if you are an adult learner.
Piano is also a lot harder to have very bad technique than other instruments since it is mostly a discrete, one-to-one mapping. If you want a chord with some particular notes, a lot of the time there is only a single fingering that will do. If you push the key hard enough, you will get the correct note, and it won't be wrong if you push harder.
By contrast, in guitar, if you push too hard, your note will be off even though you fretted in the correct place. Or, for example, everybody in guitar teaches barre chords up near the nut, when that's extremely difficult and likely to injure a beginner who has neither the strength nor control to get that right, instead of teaching barre chords near the body on fewer strings. etc.
I will say that I'm also speaking from personal experience, where I was self taught for about 12 years before taking my first lessons for my piano minor in uni. A single lesson resetting some habits I'd gotten into about hand posture and fingering reasoning and it was like a 50% increase in speed in a way that meant that songs that were near the top of ability almost felt effortless. It was wild
I'm not convinced for guitar. Some of the fastest and most famous guitarists had shockingly bad technique.
As long as you're not injuring yourself, practice and determination pretty much overcomes everything.