Speaking of Doucouré, he gave up combined events because of recurring tendinitis in his left knee, nothing else. He chose to focus on hurdling because he was afraid his body would not be able to sustain the high volume training of a decathlete. The run of injuries he suffered post-2006 proved him right in that regard.
But you are right that a lot of great athletes have been lost to other sports due to financial considerations.

Interesting to note that Doucouré basically gave up combined events in 2002 at age 19 so (except hurdles), all of his PB's were established as a 17y/18y old. I haven't got data to support this assumption but I think he would be a runaway leader of the U19 list:
10.48 (18) 46.82 (18) 13.58 (18) 6.78 (17) 7.68 (18) 4.26.53 (17)