Creating a whole new draft system is a lot of work. And creating a whole new transfer list is even more work.
And then making it so that people don't have to worry about training in high divisions means it is a recipe for disaster as people will just keep older great players and will never have to worry about training rookies.
Most people struggle to run one team, it is too complex to have to create new pages to be able to run a second team. And what if someone only wants to run one team, why should they be disadvantaged compared to someone else that has enough time to run two teams and learn twice as much?
Also creating a whole new section just for these second teams means a whole lot more stress on the server around training times and game days. And are you going to have two lots of economy pages now? Can you still train in the original league, and if so does this mean you get double training because you are training your older players instead and still receiving free high quality youngsters.
Players can have a lot of great skills by the time they are 21, imagine what happens if a div 4 team drafts it, their team will go bankrupt. And you can't make it so that the good draftees are reserved for the higher leagues as then it will simply be impossible for new teams to rise.
Sorry but this idea would never work, just too complicated and not enough benefits.