Why not make it easier for the rest, so they can enjoy the game?
Well, you could do that, but then I think you'd have a bunch of people who come and play for 3 months, and you'd probably lose the people who play for 2-3 years.
Also, I don't really get the idea that training is hard. To train players, a user can set training to 2 position, get a level 3 trainer, some guys who are 21 or younger, and the players will get better. Better enough that you'll see improvement in a single season (if you're starting at the first league level). You don't need a Ph.D. to do that. So let's not say that training is too hard for new users. If they read the FAQs and anything in the forums, they can do it. If new users cannot be bothered to spend that amount of time on the game, then I think there are few changes that could induce them to stay-unless the game becomes completely unrecognizable as Buzzerbeater.
Now, training great players is challenging, but why shouldn't players who put more thought about how to train be rewarded for that? Especially because no one is hiding information about training. The BB community is good about sharing information, and giving tips about how to build players. So, given a modest amount of effort, anyone can learn to train players.
And, now you don't think most teams need a top level SF, then why are you concerned that training guys single position is too hard (that was your point above, no?).
Now, I do think it's reasonable to ask why some new sign ups don't continue in BB, and how that may be improved.