So who is going to train one position in this scenario?
Nobody. I meant they should replace one position training with two position or maybe allow you to train an inside and and an outside skill every week so you can train guards and bigmen at the same time.
Why not disconnect training from positions completely and allow any player on the roster to be trained at the 1-position level in any skill a manager wants each week? Train your PG in rebounding, your SG in FT, your SF in stamina, your PF in handling and your C in passing if you want.
Besides, last time I checked basketball involved 5 players on the court for each team. At a minimum the fastest level of training should allow you to train 5 players per week not just 3. No wonder there is a shortage of properly trained players.
If this were a different type of game, that would be a great idea. If there was some sort of counterbalance, it could be a great idea as well.
The problem is, right now, you can choose to train or not, and there are some negatives to training (at least, assuming you're going for "optimal" training). The higher up you go in the league structure, the more those negatives weigh - in IV, for example, playing 48 minutes a game with a fresh draftee isn't much of an impediment because the player that would otherwise be there might have a sub-10k salary anyway. And even then, some choose to not train, and instead just run older rosters and use FT/GS/Stamina training with a cheap trainer and come out slightly ahead on the weekly balance sheet.
So if, instead, anyone would be trainable at any position each week, that just means that training will no longer be a choice or a cost-benefit decision, just a default "everyone does it" option. It will significantly slow down the erosion that causes higher level teams to eventually have to come back to the pack, and basically the richest teams will buy the best trainees and be able to custom-build the best new players while still competing full-bore with the best current players.
Now, of course, that's not to say that this current system is ideal either. Whenever this discussion comes up, I maintain that a better option would be to have a system where there is a training / gameshape / free throws / stamina balance, where the team's training is allocated on a percentage basis according to what you want to prioritize. That would pair up with the "anyone, anywhere" training so that the limitation on training isn't minutes played in a position, but how much you're willing to sacrifice performance. That means there's a real choice, with a real give and take, and that isn't as much of a commitment as the current one.