I would agree but the OP said that TKD in general was effective and I was pointing out that it was if you train that way. Remember in TKD some school does not teach any SD at all so there way of kicking for point would be worthless on the street. But I do agree with your above statement it goes for all Arts.
One thing I try to keep in mind is something I believe
All things being equal, strength wins
All things being equal, speed wins
All things being equal, size wins
All things being equal, technique wins
All things being equal, conditioning wins
All things being equal, determination wins
All things being equal, training wins
All things being equal, experience wins
Things are never equal
(I came up with that myself so it just makes sense to me, but if anyone disagrees, I'll listen)
But anyway, point is that I feel that the idea of SD is not a continuum, it's many dimensional. All the factors come to play and you hope you have more positives then your opponent. Any serious athlete from *any* sport is going to have advantages in stamina, maybe strength, and speed, than a non-athlete, but any brawler is going to have advantages in practical experience then they guy who trains pure SD but has no real world exposure. So who wins? Depends on how it all stacks up on that day.
Like in football, the offense has rushing, and passing, and offensive line on the rush and offensive line on the pass. The defense has pass rush and run defense and secondary. Each of the components has aspects of speed, size, and game plan. So who wins the game? Well, that's why they play it, to see whose combinations of positives wins out over the negatives.
So training TKD for self-defense is more effective along certain lines than training TKD for sport. But training TKD for sport is more effective along certain lines than not training at all, but getting in street fights is more effective along certain lines than no direct experience.
So I guess my thinking is that even badminton can be effective. They have stamina and speed and when your best self-defense move is to run like heck, that's pretty effective. Maybe not as effective in all scenarios, better than nothing