If you quit and look for a training system like BB's, the only one you will find is Hattrick. That's because in a way BB spun off from Hattrick. And Hattrick is declining almost as fast as BB.
What you.need to look at are percentages lost or gained in users.
Changes to the training system has been made fairly recently here in order to not limit users in what position they play their trainees. And I hardly think we will see drastic changes in it the way I know you would like.
And as said above there are many that like the training system as it is. And even if they did change it I hardly think we would see any difference in the long run regarding user trends.