I don't believe that everything practiced in Okinawan karate transitioned to TKD, but I do believe conditioning did exist in the early kwans. I know He Il Cho has written about it, my grandmaster Richard Chun has documented some conditioning methods in early TKD as well. In our organization, the United States Taekwondo Association, under GM Chun, some instructors do include bone conditioning and makiwara training, etc. in their training and some do not. If you look in some of GM Chun's and Master Doug Cook's books, they illustrate some of these things. But even so, it doesn't seem mainstream at all today, although I do believe it was done more frequently back in the day.
Especially nowadays, it seems that many people, at least teens and people in their 20's and 30's disregard Taekwondo because it is not hard enough for them. After working at a muay thai gym, I saw people transitioning from TKD and other traditional arts to muay thai because TKD was not offering them the hard training they desired. If we are softening up the art to make it more appealing then why are people leaving for the exact opposite reasons? They are leaving because it is too easy now! I feel as though TKD is simply attracting the wrong type of people, people who want something easy, while other styles like muay thai are attracting the people who actually want to work hard. I do not believe TKD schools who are too hardcore would not attract people. I believe that they would stop attracting the SOFT people. They WOULD attract those who actually WANT to work hard and put the effort in. Why are we choosing to attract people who want an easy, comfy art that are going to represent TKD negatively when we could be like muay thai and hard styles of karate and attract people who want to work hard and learn a real martial art?