TKD doesn't really start until the black belt level and then it becomes what you make of it. When I was kickboxing I used TKD as my base and it worked very well. I've cleaned it up and it is still my base for MMA striking. Obviously I'm the exception and not the rule but the reality is that the art is fine, it is the business of TKD that gives so many people these 'buy-a-belt' experiences.
I'm having a little trouble understanding what you mean by "TKD doesn't really start until the black belt level". Are you saying that you don't do forms and all that junk after black belt? Do all the techniques suddenly change after you get your black belt? Are there some kind of new fighting tricks they teach you once you get your black belt?
The way I'm taking it you are saying that once you get your black belt you can pretty much take what you learned and change it to what you want. If that's the case and you take things FROM TKD and make them "your own" and make them work for you, then you are not actually doing that art anymore, you are changing it to something else that's not TKD. One of the biggest fallings in styles like Karate and TKD is that it's pretty well written in stone...this is the style, bottom line. That's why I love BJJ so much...if someone comes up with a new move or something that works it's incorporated and next thing you know just about every school across the country is doing it, but you don't see that diversity in TKD and Karate....it is what it is.