I think you are being too hard on "adult beginners". I'm not sure if it is relatable but I'm learning guitar, just finished my second year self teaching it. I can't see myself as a guitar player just yet but I'm starting to have a clear perception of the stuff I have to learn and more importantly the stuff music players around me know and I'm pretty sure I can reach and overtake them. I'm thinking about "average level skills", there will always be a 8 years old kid with better abilities but I find the idea of comparing oneself to the whole world is unhealthy and far from fair. This doesn't apply only to music: the mere existance of Fabrice Bellard should prevent me from ever reaching a computer keyboard ever again.
I live in Italy, here there are two kind of musicians: 1) conservatory majors, with really really strong "fundamentals" but none to zero improvisation skills 2) other people who followed a learning path of anglo-saxon derivation, usually they have some degree of play-by-ear and improvisation skills but they show a severe lack on fundamentals skills.
By "fundamentals" I mean sight reading (meant as sight reading on first sight, everyone can read with enough time), ability to sing what you want to play in tune before playing it, strong inner sense of time and subdivision, knowledge of theory and harmony.
Side note: if you ever see musicians perform in Italy (maybe this applies to other European countries like Germany and France too) there is a very easy way to recognize if they have a classical / conservatory background: look at their feet. If they tap a foot there is a very strong chance they have no classical background as it is seen as the kind of baby wheels thing that prevents solfege from developing a strong inner sense of time.
Back on topic: as you can see there are these big two big subsets of music learning. What I'm doing is simply mix them: I study sight reading and solfege (trying to sing in tune) but at the same time I spend time transcribing by ear and following improvisation methods. There are some very strong sinergies in this: the ability to sing (in tune, not mumbling it) makes transcribing orders of magnitude easier. Same applies to knowledge of harmonic motions. Doing progressive reading exercises vastly improved my ability to play and understand odd rhythmic patterns to the point I can actually sense the lack of precision they have when I play in a garage band with my friends (not professional musicians but they have been playing for more than 20 years).
To make an even simpler example: I can play without looking at my guitar, a lot of people can't. This feel a lot like seeing people unable to type on a computer keyboard without looking at it.