#1) Well, if we're talking about a hypothetical manager doing the training, if he can't evaluate initial trainees to buy, and can't perform training effectively, then he won't do it well for a low potential trainee OR a high potential trainee. That could easily lead to a larger loss if the manager screws up a high potential trainee.
#2) Good players with builds that fit into star potential are likely to be useful for teams in D3,4,5. And there are many more of those teams than the D1 and D2 teams, and far more of them fighting for promotion. Right? 4 times as many D3 as D2 leagues, and 16 times as many D4 leagues. So there will be more possible buyers for those players (note that in USA D3, most teams roll with ~240-300K total salary, which gives ~24-30K over 10 guys- right in the star potential wheelhouse). And that doesn't even figure in the D5 teams, which may be completely filled by human managers for big countries like Spain and Italy, or ~30-50% filled in countries like USA, Poland, France. But it's possible that your location in a top league of a very small country makes you less acutely aware of that.
#3) As I tried to point out, when you consider all the costs, from initial purchase through salary for the trainee, then your net profit on a highly trained, high potential guy, isn't as much as you think.
#4) I see what you're saying here, but in this case we are just valuing the trainees themselves, and what their return will be. For the moment, stipulate for argument's sake, that the process of training the guy doesn't affect the team's ability to promote, or maintain, league position. (perhaps we assume that the trainee is going to occupy the scrimmage game slot. Then in order to show that the higher trained, higher potential guy gains more than the capped, lower potential guy, you have to show that in his 4th and 5th season, his increased salary is offset by more league wins, more league attendance, and a greater chance to promote. Like you said, salary starts to rise very quickly in these trainees, so your player better have a very impressive contribution to your team's performance to counteract that salary growth.
Also it's useful to note that in this 4th and 5th year of training higher potential guys, the star potential trainer is now 2 seasons into completing the next trainee.
I certainly know about how fast a trainee's salary can rise, as my current crop of guys will be having a substantial increase at the next season rollover. And the "discount" of low salary but better performance due to training is a great reason to train guys, whatever division level you are in. It's allowed me to have lower than league average salaries every season, yet still be able to promote or stick in my league. That extra cash can then go towards arena construction, opening up space for future salary growth.