Lower down teams do not always train for the top teams, they develop many trainees for themselves and take those trainees to the top with them. That is why talking about promotion is relevant to your argument, and where there is promotion there is also relegation.
Also, College teams cannot keep their players for the lifetimes of their careers, after their college days are over the player must move on. So you cannot equate lower division teams with college teams.
Top teams may also develop their own players from scratch just as lower teams do.
And I'll just reiterate I still don't understand why you made that (incorrect) point in the first place. So maybe you can tell me why (the hell) you raised it in a debate over the pros and cons of one or two position training.