I just don't understand why the U21 needs to be used to groom players for the NT. Being on the U21 is not a prerequisite for being on the NT. You can explain the benefits to managers of training PF/C in the fashion you suggest, sure. Some may choose to do that on their own anyway, and it seems as though some already have.
There's probably three reasons I've been having cold feet about running for this and reconsidering whether I even think my own plan is the right one, and this is one of them. Why isn't it enough to just explain how I think one sort of training is both better for a manager's own club AND also creates a potentially better NT player, and then let people decide if they think it's a good plan or if they'd rather train the more traditional way? And I don't have a good answer for that.
I think what got me into this was that idea that the sort of bigs U21 requires are both:
-not sustainable for the majority of the managers training them and not particularly salary efficient for their clubs either
-not easily trained in secondaries later on, and therefore not really providing the NT with the best return on our high potential players.
So, it felt like U21 priorities were costing us a precious resource (good trainees and the managers who put in the work to train them) and I wanted to see a plan to re-align what U21 was about to something that didn't cause those conflicts. But I know it's not a perfect solution.
Maybe a better solution is something closer to what Jelme has already suggested: simply more communication to managers about the financial implications, and the possible benefits of choosing to train their player in a balanced way with a focus more on making the NT vs the U21 team.
If every initial U21 email to a potential trainee included "here's the potential draw backs to training your player this way and here are some other training possibilities" maybe that is enough to get some people to consider training for the NT instead, while still leaving U21 free to play whatever players happen to be best suited for winning any given season?