I just plainly reject the notion that throwing a league game by playing 3 trainees out of position simultaneously is a reasonable option or makes things worse compared to what they are now.
People will have the possibility to juggle training having substantially more flexibility (i.e. you can alternate out of position training and best position training when needed because what you do with one trainee does not affect the other trainees). If anything, I'd think people would try and give training in the cup/scrimmage game, which are less important, but even then it would come at the risk that they can't be sure to give enough minutes to the trainees. The problem you're insisting on would also be a concern for the initial 4-5 weeks of competitive cup games at best and many of such games are very unbalanced anyway; after that people would throw scrimmage games not competitive games.
You're happy to criticise the fact you could throw 1 game by playing all your trainees, but you're fine with the current system where you have to potentially throw 2 games to train out of position? And let's be clear that for the rest of the league throwing a game now or in the new system would be exactly the same thing. There is also the impact of additional training flexibility on GS training and the hard-to-create players will still be hard to create.