Wow, you really covered a lot of ground. I, however, disagree with many of your points, although I believe they hold validity.
For example.
Point 1. TKD is relatively new but it's base is not. The opinion that TKD is a relatively new art; while BJJ you believe is very old because of it's roots. Well TKD's roots trace from Shotokan karate... which traces a back to Okinawan karate... which extends back to the 14th century is it (I'm not a karate history academic so feel free to comment/edit.)? And IMHO, BJJ is a 'modernized' form of Judo, with the modernization seeking to address the conventional Judo de-emphasis on ground work. As well as certain practicality in among technique. So both BJJ & TKD have a 'modern' genesis; yet hold eastern traditions that extend back many centuries.
Am I a fan of TKD..? me no. When one takes a broader view of karate tradition, karate, hence it's descendant TKD, broaches much more than the straightforward observations voiced here.
Point 2. The forms ae practiced effectively for the sake of getting good at the forms and preserving them (FALSE). When I stared karate, kata was emphasized as an important part of the curriculum. Important for what..? Well karate is martial art, and it's application self defense... so kata is important for developing what? Dunno?
I understand that many people question / forms or kata or poomsae, etc. ... although at all the dojos I have attended, their relevance was never questioned. How about answering the question of kata with the precept of 'understanding.' And apply the same to TKD. Just because TKD, certain orgs, stress a certain kind of competition utilizing kicks, doesn't that benefit the practitioners? Do that kind of competition eclipse the art... so that I have to say, cross train Muay Thai?
My position. I don't feel I have to train Muay Thai for squat. Is Muay Thai a formidable martial art... sure is? TBT though, karate tradition has an answer for everything... if one thinks about it. Far from any name on any competition circuit, I have defeated boxers, wresters, kickboxers, judo players, other karateka / 'master' level, including many instructors. Why? Because they didn't understand the worth of tradition, eastern martial art tradition. It comes back to understanding.
The progression is martial arts isn't that the Gracie's customized Ju Jitsu,,, 'modernized' Judo. Judo always had ground work... just became less developed technically. Judo throws do something different than BJJ takedowns,,, which people complain about BJJ's takedowns not being 'effective' (false). The only progression is understanding.
TKD is, in fact a complete art, if you think about it. Realizing that is how I became able to defeat the MMA 'functional,' popularized styles like boxing & Muay Thai, etc.
