I want to have the chance of training a rookie and sell him when he is good enough, or keep him when he boosters my team.
Nothing about my proposal changes this.
If you are going to keep a draftee and train him, fine. However, you have limited training spots, so you will have to replace one of your current trainees with a new draftee. Sell the old trainee; keep the draftee. You cannot do both.
As a practical matter, I find myself training 4 players at full time (48 or more minutes) and 2 at whatever is left over (24 to close to 48, depending on the weird choices of the substitution engine). So when 3 draftees come in, I have to decide which of the three draftees is worth training, and which of the current trainees will have to be sold off for whatever the market will bring.