Plus in the TKD system, they even confess your actual teaching starts at black belt, so you arent technically climbing the tree until you are at the top of the tree and that stands contradictory to them putting a self defence marker on it. Useless for self defence if it takes you 20 years to learn it
If that's your understanding then it's either been very badly explained or you've misinterpreted...
Firstly, the only way I can see it taking 20 years to reach black belt is if you're either very very bad or you do like 10 lessons a year...
Black belt is not top of the tree.
Your proper training starts when you start, but until you get to first Dan you're a beginner - first Dan is 'novice'. That's the point where you (should) have a good understanding of the fundamentals and are in a position to build upon those.
If you can't apply what you get taught at your first few white belt lessons then that's on you I'm afraid.