One of the issues with kollelim that I have seen is that eventually guys in kollel want to move on and become a Rebbe, shol umashiv etc due to parnassah but also due to the fact that at a certain age people begin to get more of a sipuk teaching and giving over Torah than just learning all day.
The problem then is that the Rebbe job market is ridiculously flooded and yungleit often have trouble finding a job in chinuch. They now have been out of high school for say 10+ years and have no skillset to get high paying secular jobs. For them to start college at that age is hard especially with kids. The transition out of kollel is rough.