I disagree. I played guitar for about twenty years and, because I ignored proper technique for fretting in favor of doing what felt more comfortable and what I thought looked cool, I ended up severely damaging my wrist where now I can’t play guitar for longer than about fifteen minutes without being in some pretty gnarly pain.
Also, it’s not just 10,000 hours that makes you good. It’s 10,000 hours of actively trying to get good and one of the ways that happens is by not building roadblocks for yourself down the road with bad technique.
Regarding the thumb thing, I usually hang it over the fretboard unless I require more intricate chords where the leverage of my thumb on the neck would help out. You can see something similar in the guitar world: Dimebag from Pantera would play riffs with his thumb over the neck and then solo with it behind the neck for more reach.